feat: cherry-pick community web UI improvements

[phona]

Cherry-picked Community PRs

High Priority (Mobile & UX fixes)

1. fix(app): respect safe area in web titlebar anomalyco/opencode#35481 — iOS PWA safe area in titlebar (4 lines)
2. fix(app): insert newline on Enter for mobile prompt input anomalyco/opencode#32830 — Mobile Enter key inserts newline instead of submitting (10 lines)
3. fix(app): eliminate session switch latency — deferRender, Suspense flash, instant rendering anomalyco/opencode#31395 — Eliminate session switch latency by removing deferRender (45 lines removed)
4. fix(core): add diff size limits to prevent UI freeze with large changesets anomalyco/opencode#35546 — Diff size limits to prevent UI freeze with large changesets (248 lines)
5. fix(app): clipboard fallback so copy buttons work in non-secure contexts anomalyco/opencode#17549 — Clipboard fallback for HTTP (non-HTTPS) contexts (89 lines)

Medium Priority (Performance & PWA)

6. fix(app): prevent web client freeze from delta event bursts and SSE reconnect loops anomalyco/opencode#33289 — SSE sleep abort-aware to prevent reconnect delays (SDK-level fix)
7. feat(app): add offline PWA support anomalyco/opencode#33532 — Offline PWA support with Workbox service worker (380 lines)

Skipped

• feat(app): add "Send with Ctrl+Enter" option anomalyco/opencode#13637 (Ctrl+Enter setting) — Skipped due to massive i18n file conflicts from recent upstream translation PR. Can be cherry-picked later when i18n stabilizes.

What each PR fixes

PR	Issue	Fix
anomalyco#35481	iPhone notch overlaps titlebar buttons	env(safe-area-inset-top) padding
anomalyco#32830	Mobile users can't type newlines	Touch devices: Enter = newline, send button submits
anomalyco#31395	White flash on session switch	Remove deferRender gating
anomalyco#35546	UI freezes on 10000+ file diffs	Truncate large diffs with configurable limits
anomalyco#17549	Copy buttons fail on HTTP	document.execCommand("copy") fallback
anomalyco#33289	SSE reconnect delay on abort	Sleep function checks abort signal
anomalyco#33532	No PWA install prompt, no offline	Workbox service worker + precaching