feat: rebuild window tags and pinned sessions

[PathGao]

Summary

Windows can be given a named color identity, which is reflected in the native title and cross-window destination list. Pinning an identity saves the window's file set at close; the HOME page can reopen that set and unpin it.

Rebuilds the functional scope of #226 without its stale merge history. Depends on #279; merge after it.

Validation

• npm run check
• npm test (125 passed)
• cargo test (20 passed)

[PathGao]

This rebuilds the functional scope of stale draft #226. Its branch is directly based on #279; please merge it after #279. Validation: npm run check, npm test (125 passed), and cargo test (20 passed).

[PathGao]

Updated after the latest chain rebase: the Home menu now has a dedicated window-organization group immediately below export actions. It exposes Window Tag… (create or edit the current window tag) and Merge All Windows Here; a regression test locks their order. npm run check, npm test (126 tests), and cargo test (20 tests) pass locally.

[PathGao]

@alecdotdev As the stack is almost through, could you also take a look at #230 and #281 when convenient? They capture two follow-up product decisions: the YouTube embed fallback/compatibility direction and the minimal macOS app-menu boundary.