feat(web): add favorite thread markers

[HabsaTheDog]

What changed

• Add an Add to favorites / Remove from favorites action to thread context menus in both sidebar versions.
• Persist favorites locally by scoped thread key, including sanitization during hydration.
• Show a filled amber star immediately before the existing timestamp or status slot.
• Keep the marker visible while the row's timestamp/status yields to hover and focus actions.

Why

There was no lightweight way to visually mark an important thread for later. This keeps the feature local to existing UI state and avoids server, database, or protocol changes.

Fix

The initial marker was nested inside the timestamp/status element, which intentionally fades out on row hover and focus. A context-menu interaction leaves the row in that state, so the favorite toggled correctly while its star remained hidden. The marker is now a sibling immediately before that transient slot in Sidebar V1, Sidebar V2 cards, and Sidebar V2 slim rows.

User impact

Right-clicking a thread now toggles a persistent favorite marker that stays visible, including immediately after the context-menu action. The marker survives app restarts and uses the existing amber palette in light and dark themes.

Validation

• Targeted formatting and lint for both sidebar components
• vp run --filter @t3tools/web typecheck
• vp test run apps/web/src/uiStateStore.test.ts (16 tests passed)
• Original web unit suite validation (201 files, 1,766 tests passed)

Generated with Codex (GPT-5.6) in T3 Code via the Codex harness.

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