Status messages time out, errors wait for you, and Messages lists what you missed #592

Context

TuiCode's status bar keeps replies like Saved: … until something replaces them, and loses errors at the next tab switch. This moves it onto MentalDesk.Tui's AppStatusBar and MessagesDialog, so it follows the style guide's Status messages rule and gets a Messages list. Pitch: #587.

It needs two additions to AppStatusBar in mentaldesk/tui-style-guide first, and a MentalDesk.Tui release carrying them: resting text, and recording a message without showing it. They're specified in mentaldesk/tui-style-guide#108. That repo is in this team's contributesTo, so the Dev opens that PR itself (closing #108, Needed by mentaldesk/TuiCode#592), then waits on its release of MentalDesk.Tui before building this.

Acceptance criteria

• After Ctrl+S, Saved: <file> shows on the status bar and goes after 5 seconds, or at the next key if sooner. The same for Copied 3 lines, Renamed: …, Opened folder: … and Review submitted on #….
• With nothing to say, the bar shows the active file's full path, or TuiCode • F1 help with no file open, as today.
• Something that happens by itself (a folder reloaded, a file changed on disk) shows for 5 seconds, then the path comes back.
• Stopping the C# language server shows C# language server stopped: … once; when it restarts, the message comes off straight away if still showing.
• An error (e.g. Couldn't refresh the review with the network off) shows in the error colours and doesn't time out. If it happened by itself while I was typing, it's still there after 5 seconds and goes at my next key after that.
• An error that happened by itself adds to a ⚠ n button just left of the grammar, in the error colours; log warnings written since I last looked count there too.
• Clicking ⚠ n, Help › Show messages, the palette's Show messages, or mnemonic sm opens Messages: this session's messages, newest first and selected, each with its time, errors in the error colours, log warnings among them. Opening it clears ⚠ n.
• In Messages, Ctrl+C copies the selected line and Esc closes it; both are buttons on its bottom border.
• Help › Show log (sl) still opens the log, and no longer has a counter of its own.
• The grammar, Column select, Wrap, the diff's File 2 of 7 • Change 1 of 4 and Ln, Col sit together at the bar's right end, and a long message no longer pushes them off.
• The focus word (Editor, Explorer, …) and a pending chord (Ctrl+G…) still show at the left as today.

Tests

• Every existing SetMessage/SetError call site is sorted into reply, news or error; a test per category on a representative caller (save, file-changed reload, review refresh failure).
• News withdrawn when it stops being true: language server restarted, review refresh succeeded, reload finished.
• Resting text follows the active tab and the no-file state.
• Log warnings recorded into history and ⚠ n without being shown; Messages clears the count.
• The ShowCopy timer is gone and copy uses a reply.
• sm is free and no other mnemonic's prefix; Show messages sits next to Show log in the Help menu.
• Existing status-bar tests (focus word, chord, diff status, position) still pass against the new bar.

Out of scope

• A setting for the 5 seconds.
• Keeping Messages across sessions.
• Flashing log warnings on the bar as they happen.
• Moving loading indicators (already off the bar, Open pull request, Submit review, the Review pane and Compare to revision show loading, empty and failed inside themselves #525).
• Any other change to the library beyond bumping to the release with the two additions.