feat(puter-js): puter.notifications

puter.notifications in puter.js. Part of PUT-2170. Stacks on #4351. Replaces #4322.

What

• There's no onPush, and apps never choose between live delivery and push; the server decides.
• Clicks come only from the desktop's notificationClick broadcast. A click that arrived before the first handler goes to that handler once, after on() returns.
• onNotification needs an app context (app_required otherwise) and live notifications on the server (events_disabled otherwise).
• Typings in index.d.ts, a docs page per method with sidebar entries, and a playground example.

Deployments without a push service. The /push/devices routes come from an extension and are absent on a plain self-hosted install. The device methods turn that 404 into push_unavailable ("Push notifications aren't available on this deployment"). A missing device is device_not_found, so the two can't be confused. Documented on each method.

Tests

• Unit (mocked fetch): 58 passed, including push_unavailable for each device method, a non-JSON 404, and device_not_found passing through.
• API suite on node against a core server with no push service: the notifications and perms cases pass. The device cases that needed a push service were removed from this suite, along with its push capability.
• check:puterjs:types is clean. Typecheck: no new errors.

Not verified

Clicks from a real desktop are covered by unit tests only.