Move SFTP and S3 into the tab strip, and the host list into a card grid

Four asks in one pass over the desktop head, and two of them are furniture
moving rather than anything new.

THE STRIP IS THE WINDOW'S NOW, not the terminal's. Vaults, SFTP and S3 sit at
its head and cannot be closed, and open terminals follow them. SFTP and S3 left
the nav rail to get there, which is the one semantic change: they are the two
destinations you stay in while something runs, and a rail entry is for
somewhere you go and come back from. So the rail belongs to the Vaults tab and
collapses with it, which also means SFTP, S3 and a terminal each get the full
1016 rather than the 826 a page gets.

The tab is expressed as "a page, and not one of the two the strip took" rather
than as a fourth ShellSurface. Both are still ShellScreen members and have to
be — that is what they are on the phone, where they are two rows in a hub — so
a surface each would have been a second way to say a thing Screen already says.
IsTransfersShowing and IsBucketsShowing light the other two tabs unchanged.
What is new is one field: the page Vaults comes back to, because it is the one
tab with sub-navigation and therefore the one that can return to the wrong
place. That is not the hidden field ShellSurface argues against — that one
would be a second copy of "which page"; this is a tab remembering its own.

THE HOSTS SCREEN IS A GRID, and the 268-pixel sidebar is gone. That column was
choosing among forty machines and editing one of them at two-thirds width, and
it was narrow so the editor beneath it could be a column at all. Cards took the
first job at full width; a 304-pixel drawer took the second and collapses when
nothing is selected. Pressing a group card narrows the grid; SHOW ALL is the
way back. The group editor moved into the drawer as well, which finally makes
IsEditingGroup mean the same thing on both heads — it was the phone's alone,
because the desktop's editor was a bar that was always on screen.

AreHostsExpanded and ToggleHosts went with the control that used them. They
folded the whole list away under one heading, an affordance that existed
because the column was narrow. Folding a single group is a different thing and
is still here.

THE TYPE SCALE IS A POINT LARGER and the text ramp is white. The base size was
never stated anywhere — a bare TextBlock took TextElement's default of 12 — so
raising the scale meant naming it, on Window and on UserControl. The second
selector is not redundancy: the layout harness hosts a UserControl in a window
it builds itself, and without it the suite would measure every screen a point
smaller than it ships, silently. A selector on TextBlock would have been the
obvious way and is wrong, because a style setter beats an inherited value and
would collapse every deliberate step back to one number.

#E3E7F4 is a blue-tinted white on blue-black surfaces, which costs contrast
twice — once for being darker than white and once for sharing a hue with what
it is drawn on. Pure white is 18.3:1 against the canvas where that was 15.5:1.
Every step below moved with the top, so the intervals the design chose are
kept and TextDim clears 9:1 against 6.4:1. The palette is shared, so the phone
has both changes too.

TWO DEFECTS THE HARNESS STRUCTURALLY CANNOT SEE, found by rendering the screen
rather than by measuring it, and both now covered.

The tile was 232 and was first written as 248, from arithmetic that left out
the scrolling stack's own margins. Every layout test passed — the harness asks
whether a control is inside the window, never how many fit on a line — so the
grid quietly became one column wide at exactly the minimum this application
guarantees, which is the shape cards exist to avoid.
TheHostsGridKeepsTwoColumnsAtTheMinimumWithTheDrawerOpen counts columns
instead, and fails at 248.

And a card's text ran past its own border, because a horizontal StackPanel
measures children with infinite width: a TextBlock inside one never learns it
is short of room, so TextTrimming never fires. Both card rows are grids with a
star column that gives way and an Auto column that does not — a hostname with
its tail cut is still the machine you were looking for, where a badge or the
word naming an auth method is not.

The keychain header changed shape for the same class of reason. It was
Auto,Auto,*,Auto with the buttons last, so the slack column was the only thing
absorbing a change of width and five buttons fell off the right edge the moment
the type grew. That is how GENERATE lost the word KEY once already. The summary
sits in the star column and trims now, so the buttons always get their width.

HostSidebarTests became HostGridTests and moved to the grid with the gestures
it drives. docs/design-import-gaps.md gains a v3 section naming the five
toolbar controls in the design with nothing behind them — a view-mode switch, a
tag filter, a calendar, a share control and Serial — and manual-checks.md and
the README follow the controls that moved.

src/DodoSSH.Client.Shell/ViewModels/MainWindowViewModel.cs

@@ -893,6 +893,58 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
     [RelayCommand]
     private void ShowTerminal() => Surface = ShellSurface.Terminal;
 
+    // ---- The desktop's fixed tabs ----
+
+    /// <summary>
+    /// Whether the tab strip's <c>Vaults</c> tab is the one showing.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The desktop strip holds three tabs that are always there — Vaults, SFTP, S3 — and then a tab per open
+    /// terminal. This is the first of the three, and it is the only one with anything under it: the nav rail
+    /// and whichever of its screens the rail points at. So the rail is drawn on this and nothing else, which
+    /// is what the strip buys — a rail beside a file transfer would be offering nine destinations none of
+    /// which is the screen you are looking at.
+    /// </para>
+    /// <para>
+    /// Expressed as "a page, and not one of the two the strip took" rather than as a fourth
+    /// <see cref="ShellSurface"/>. SFTP and S3 were already <see cref="ShellScreen"/> members before they
+    /// were tabs, and they still are on the phone, where they are two rows in the hub rather than two tabs —
+    /// so a surface for each would have been a second way to say a thing <see cref="Screen"/> already says,
+    /// and the two would have had to be kept in step. <see cref="IsTransfersShowing"/> and
+    /// <see cref="IsBucketsShowing"/> are the other two tabs, unchanged and already used by both heads.
+    /// </para>
+    /// </remarks>
+    internal bool IsVaultsTab => IsShowingPages && IsVaultsPage(Screen);
+
+    /// <summary>The pages that live under the Vaults tab, as opposed to under SFTP or S3.</summary>
+    private static bool IsVaultsPage(ShellScreen screen) =>
+        screen is not (ShellScreen.Transfers or ShellScreen.Buckets);
+
+    /// <summary>
+    /// Which page the Vaults tab returns to.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The Vaults tab has sub-navigation and the other tabs do not, so it is the one tab with somewhere to
+    /// come back to: leaving the keychain for SFTP and pressing Vaults again should land on the keychain,
+    /// not on the hosts screen. Without this it would land on whatever <see cref="Screen"/> happened to hold,
+    /// which after a visit to SFTP is <see cref="ShellScreen.Transfers"/> — a Vaults tab showing the file
+    /// screen.
+    /// </para>
+    /// <para>
+    /// <b>This is not the hidden field <see cref="ShellSurface"/> argues against</b>, and the difference is
+    /// worth stating because the two look alike. That one would have been a second copy of "which page",
+    /// kept because the enum could not hold two facts at once. This is the Vaults tab's own state — a tab
+    /// remembering its page, the way any tab does — and nothing else reads it.
+    /// </para>
+    /// </remarks>
+    private ShellScreen vaultsScreen = ShellScreen.Hosts;
+
+    /// <summary>Selects the Vaults tab, on the page it was last left on.</summary>
+    [RelayCommand]
+    private void ShowVaults() => ShowScreen(vaultsScreen);
+
     // ---- Open terminals ----
 
     /// <summary>
@@ -2475,6 +2527,12 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
     {
         RaiseSurfaceState();
 
+        // What the Vaults tab comes back to; see the field.
+        if (IsVaultsPage(value))
+        {
+            vaultsScreen = value;
+        }
+
         // Read when the screen is opened rather than kept in step with every sync pass. Two full logs is
         // thousands of decryptions, and nobody is waiting for their own connection from an hour ago to
         // appear on a screen they are not looking at. Not awaited: navigating must not block on a read.
@@ -2559,6 +2617,7 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
         OnPropertyChanged(nameof(IsBucketsScreen));
 
         OnPropertyChanged(nameof(IsShowingPages));
+        OnPropertyChanged(nameof(IsVaultsTab));
         OnPropertyChanged(nameof(IsHostsShowing));
         OnPropertyChanged(nameof(IsTransfersShowing));
         OnPropertyChanged(nameof(IsVaultShowing));