Compose shared dialogs, tooltips and navigation

[mahanti]

Buzz now has shared Base UI dialogs, tooltips, tabs and navigation rows. Shared components own focus, dismissal, keyboard behavior, semantic colors and overlay layers so screens can reuse them.

Stack 3/4: #73 → #85 → #86 → this PR → #88. Base UI and public fonts/assets remain; no private dependencies are added.

Dialogs explicitly expose their modal state so Buzz's existing shortcut guard blocks background navigation. The mounted regression asserts that marker while retaining pending/dismissal coverage. Tooltips keep accessible names and descriptions; tabs associate triggers with their panels.

Validation: all 52 design tests, TypeScript, design guards and viewer build passed. The completed app stack passed the dialog shortcuts and thread-history journeys in Chromium and WebKit. A saved CI trace showed the channel's unread badge arriving during the history test; its selector now accepts that accessible name while retaining every history and DOM-preservation assertion. No browser cases are added or removed.

A separate relay test-only commit establishes signer callback order before asserting the shared cooldown. Real cryptographic preparation can complete out of order; the test now controls that ordering without changing the production service, timeout or cooldown assertions.

Hosted CI is required on the final head. Full local just scan, native visual testing and human review remain deferred; this PR is draft.