feat(notifications): add mark-as-unread session action

[wen2zhou]

Problem

A Session that has already been acknowledged cannot be deferred from its sidebar menu. Users need a quick way to return its latest Messages entry to the unread queue without reopening or changing the Session.

Proposed change

• Add an eye-icon Mark as unread action to the desktop and mobile Session action menus.
• Add the typed renderer, Electron IPC, and local/remote Web command path for marking Sessions unread.
• Transactionally restore the latest valid notification for each requested Session to unread and promote its stable notification identity across the bounded Messages snapshot window.
• Refresh the shared notification store and badge after the mutation.
• Add renderer translations for Spanish, French, Simplified Chinese, Traditional Chinese, Japanese, Korean, and Russian.

Scope and non-goals

• Only the latest valid notification for a Session is marked unread; older history remains read.
• Sessions without an existing Messages notification do not get a synthetic activity item.
• No database schema, migration, native notification delivery, or Session persistence format changes.

Acceptance criteria and validation

All listed checks ran after the final material edit and after rebasing onto origin/main at cb795e8.

• Session menu rendering and action wiring -> npx vitest run with WorkspaceSidebar.render.test.tsx -> passed.
• Latest-notification unread persistence and bounded snapshot promotion -> notification-inbox-repository.test.ts -> passed.
• Renderer store, controller, IPC, host command, preload, renderer-contract, surface inventory, and i18n consumers -> targeted 12-file Vitest set -> 952 tests passed.
• Node and Web type safety -> npm run typecheck -> passed.
• Source lint -> npm run lint -> passed.
• Final impact selection -> npm run test:affected -- --base origin/main --head HEAD -> unknown host-command module ownership failed closed to the complete portable suite; 21,997 tests passed and 238 were conditionally skipped.

The exact-head full portable suite covered the cross-process consumers. No platform-specific native behavior changed, so packaging and Electron E2E lanes were not run locally. No independent code review was performed locally; PR review and CI remain authoritative.

Review focus

• The transaction in NotificationInboxDbRepository preserves notification id, dedupe key, content, and action state while assigning a fresh sequence so an older marked-unread item re-enters the bounded Messages snapshot.
• The new action remains in the Session-management menu group and applies consistently to desktop and mobile navigation.
• Request validation and contracts remain identical across Electron and Web transports.