feat(desktop): multi-KB picker — switch between projects

[plind-junior]

What you're trying to do

Today the AppImage hardcodes $HOME/.vouch/ (or $VOUCH_KB_PATH). Real users have a .vouch/ per project; vouch-the-CLI walks up from cwd to find it. The desktop app needs an explicit picker so a user can keep three projects open in sequence without env-var dance.

Suggested shape

File → Open KB…             (folder picker — must contain .vouch/)
File → Recent KBs ▸          (last 5, persisted in ~/.config/vouch-desktop/recent.json)
File → New KB…              (folder picker, runs `vouch init` then opens)

• Recent KBs list lives in ~/.config/vouch-desktop/state.json (xdg-config-home compliant).
• Window title updates to reflect the active KB.
• Switching KBs restarts the python sidecar against the new root; in-flight requests cancelled.
• Refusing to open a folder without .vouch/ shows an inline error with a "create KB here" button (calls vouch init).

Acceptance

• A user can switch KBs from the menu without restarting the app; the queue re-renders against the new KB within 2s.
• Recent-KBs list persists across app launches.
• New KB… against an empty folder produces a usable KB (verified by vouch status returning a row for the starter claim).

Out of scope

• Side-by-side multi-KB views in one window — one window, one KB in v1.
• Cross-KB search (federation territory).