feat(team-inbox): unified team inbox

[Neonforge98]

Summary

This started as a task to extract the team-inbox feature from #530 onto a clean develop. That premise turned out to be stale: develop already carries its own, more advanced team-inbox lineage (#521 merged, reverted, re-added via 3fd19ba4f, then extended three more times through 9238b2cb0 — session handoff, collaborative work item threads — none of which #530 has). Porting #530's snapshot over that would have been a regression, not an extraction.

So this PR does not port #530. It closes four real gaps found while diffing develop's current team-inbox implementation against #530 and against itself:

• Orphaned IPC registration (the actual blocker). team_inbox_list_page / unread_count / mark_read / mark_all_read / mark_unread have existed in the project-management crate and been called by the frontend since the post-feat(team-inbox): add unified team inbox #521-revert re-add, but handler_list.inc never got the registration back. Every Team Inbox call has been rejecting at the Tauri boundary as an unknown command — the feature has been non-functional on develop.
• Read-state idempotency lived only in the view. TeamInboxView's readAt guard protected its own handlers but not the coordinator itself, so any other caller could double-fire markRead/markUnread and trigger a redundant local/cloud round trip. Moved the guard into teamInboxCoordinator.setReadState, checked against live cache state rather than a caller's snapshot.
• No forced resync after a failed optimistic mutation. The coordinator already reverts an item's own optimistic readAt on failure, but nothing resynced the list/badge against server truth afterward, which risked drift if the revert raced another concurrent update. Extracted the success/failure/resync orchestration into a pure, framework-free helper (teamInboxReadTransitions.ts — repo policy is no .tsx unit tests) and wired handleSelect/handleMarkRead/handleMarkUnread through it.
• i18n gaps. The unread sidebar badge's aria-label was a hardcoded English template literal (now uses the teamInbox.unreadCount key that already existed in common.json but was unused); the teamInbox namespace itself only existed in en/zh — added to the other 11 shipped locales (de, es, fr, ja, ko, pl, pt, ru, tr, vi, zh-Hant), mirroring en's key structure.

Checked and found already correct, no change needed: markAllRead's unread-count bookkeeping already derives from the DB-wide bulk result and the server's mention-read response (not a loaded-page subtraction), and en/zh common.json have no duplicate teamInbox key or duplicate loadMore key — those specific defects were unique to #530's history and aren't present on develop.

Supersedes the team-inbox portion of #530.

Commits

• fix(team-inbox): register orphaned IPC commands
• fix(team-inbox): guard read-state idempotency in the coordinator
• fix(team-inbox): revert and resync optimistic read state on failure
• fix(team-inbox): translate the unread sidebar badge aria-label
• feat(team-inbox): add teamInbox namespace to remaining locales

Validation

• tsc --noEmit --pretty false: clean
• vitest run (TeamInbox + sidebar menu suites): 16 files, 89 tests passed, including 5 new tests for the read-transition helper and 2 new tests for the aria-label wiring
• eslint (all changed files): clean
• cargo check -p org2: passed
• cargo test -p project_management team_inbox --no-fail-fast: 14 passed, 0 failed