Give hosts and terminals their own screen, and the rest of the vault another

Rebuilds the client's shell from an imported design: a titlebar and nav rail
it draws itself, real multi-session tabs over the one WebView, a Ctrl+K host
search, and a vault screen that merges keys, passwords and pinned host keys
into one table. Hosts left the vault column for their own screen beside the
terminal, which is what the design asks for and turned out to be the better
split anyway.

Two screens the design shows have nothing behind them yet — file transfer
and teams — and say so plainly rather than rendering invented data; every
other gap between the design and this build is recorded in
docs/design-import-gaps.md.

README.md

@@ -44,6 +44,12 @@ Three consequences worth knowing before you read further:
 The reasoning behind each major decision is recorded in [`docs/adr/`](docs/adr/), starting with
 [the E2EE trust model](docs/adr/0001-e2ee-trust-model.md).
 
+The desktop client's interface was built from a design covering more product than exists yet — file
+transfer, teams, saved snippets, port forwarding. Everything that design asked for and this build has not
+got is written down in [`docs/design-import-gaps.md`](docs/design-import-gaps.md), with the layer each
+piece would land in and what the interface shows in its place. Nothing was rendered with invented data to
+fill a screen.
+
 ## Repository layout
 
 ```
@@ -64,6 +70,7 @@ src/
   DodoSSH.Client.App       Avalonia; the only project that knows about a UI toolkit
 tests/                     one test project per source project
 docs/adr/                  architecture decision records
+docs/design-import-gaps.md what the client's design asked for and this build has not got
 ```
 
 Everything under `src/DodoSSH.Client.*` except `App` is deliberately free of Avalonia. That is the