[doskoi]

Closes #704.

Add opt-in browser notifications when an agent finishes responding, needs input, or an experiment run finishes/fails. Notifications cover all projects, including chats outside the selected project or paginated sidebar, and work while the dashboard is focused or in a background tab/window. Clicking opens the relevant chat or project.

This refreshes the notification implementation proposed by @rdereparadores in #359 against current main, with attribution retained. It also adds:

• An initial chat baseline and reconnect snapshots so old results stay quiet and missed completions can be recovered.
• A bounded delivery ledger coordinated with Web Locks so multiple tabs do not each sound the same notification.
• Per-reply identities, branch-selection suppression, queued-follow-up checks, retries, and guards against notifications from a retired workspace.
• Browser-local switches, permission/secure-context handling, a test button, and strings in all nine current locale catalogs.

The UI consumes the existing global event stream and uses relative application URLs. No hostname-specific configuration, backend changes, extra notification service, or new package dependencies are needed. At least one dashboard tab must remain open; this does not add browser-closed push delivery. Permission and preferences are scoped to the browser origin.

Validation:

• UI suite: 333 tests pass, including 18 notification tests.
• pnpm build: localization checks, route generation, TypeScript checks, and production build pass; regenerated ui/dist is included.
• Style checks, 9 dev-slot helper tests, and 10 Python helper tests pass.
• Firefox 150: built UI settings, one shared event stream per tab, background completion, two-tab deduplication, reconnect recovery, test notification, and click navigation pass. Notification calls were captured for these automated checks; OS popup rendering was not asserted.
• Rust and platform gates were not run locally because this environment has no Rust toolchain; awaiting upstream CI. No Rust source or dependency files changed.

The new translations outside English and Simplified Chinese are machine-assisted and would benefit from native-speaker review.