feat(notifications,uploads): delete a notification; size images to server caps

[Adron]

⚠️ Stacked on #40. Merge order: #38 → #39 → #40 → this.

Summary

The two papercuts from Thread A: P1 (delete a notification) and P3 (adopt GET /api/limits).

P1 — delete a notification

DELETE /api/notifications/:id exists and is documented; iOS only had read and mark-all-read, so a notification could never actually be cleared from the tray.

• APIClient.deleteNotification(id:) against the 204 route.
• Swipe-to-delete in NotificationsView, applied optimistically: the row goes on swipe and returns only if the server rejects it. The failure alert is a separate channel from errorMessage, which drives the full-screen empty state and would have swallowed the message entirely.

P3 — stop sending 2.9× the pixels the server keeps

The client resized uploads to 2048px per side. The backend resizes every image to 1200px / 1.4MB regardless — messages, avatars and document images all route through resizeAvatarToLimit. Every photo posted carried roughly 2.9× the pixels the server kept, for nothing.

• New ServerLimits model + APIClient.serverLimits() against GET /api/limits (public, no auth), held for the process by ServerLimitsStore.
• RootView refreshes it once at launch. Failure is not an error path — the fallback caps are the values the deployed backend reports today, so an upload is never blocked by a missing fetch.
• ImageUploadProcessor takes the caps as a parameter instead of hardcoding them and derives its ladder from them: the server cap leads, rungs at or above it are dropped. maxPixels: 1200 → [1200, 1000, 800]; a raised server cap is honoured with no client release.
• The store is read off the main actor (ImageUploadProcessor runs in a detached task), so it's lock-guarded rather than actor-isolated.

Testing

861 tests, 0 failures (838 → 861).

• APIClientLimitsTests — route, no-token path, full decode, and a payload missing the optional video/message sections.
• ServerLimitsStoreTests — fallback before refresh, adoption after, fetch failure keeping usable caps, and 50 concurrent reads during a refresh (it's read from a detached task, so this matters).
• ImageUploadProcessorTests pinned the old constants; they now pin the derived ladders and pass explicit caps, so results no longer depend on whether GET /api/limits answered during the run. Added: a 4000px photo comes out at 1200, and a raised cap produces a larger image.
• APIClientNotificationsTests — delete path, bearer, percent-encoded id, 404 and 401.

Note for whoever runs the suite next

This run initially reported a bogus failure: Threads B and C were testing concurrently on the same simulator UDID and DerivedData, which killed my runner mid-suite and restarted it into Thread B's test bundle (AIArtifactTests showed up in my log). Re-run on a dedicated UDID + -derivedDataPath, it's clean. CLAUDE.md now says so under Build & Test.

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