feat: make full sessions reliable on mobile

[leoncheng57]

Summary

• preserve a reader's transcript position during live output and expose an accessible Jump to latest affordance only for appended or growing rows, not status-only idle transitions
• give the 390px composer a full-width 16px multiline textarea, compact native reminder picker, 44px Attach/Send targets, safe-area padding, and code-friendly input hints
• contain hostile markdown, stack the Changes rail on mobile, prevent pull-to-refresh overscroll, and keep SSE recovery retrying with capped backoff
• retain unchanged event identities and memoize transcript rows/markdown to reduce long-session rendering work
• integrate current main while preserving recent-session recording and issue bug: Build sessions can lose shell and edit tools after a mode switch #15 permission-policy activation/concurrency behavior

Verification

• npm run typecheck
• npm test (153 passed)
• npm run build
• lockf -k /tmp/custom-dca-opencode-e2e.lock npm run test:e2e (117 passed)
• git diff --check

Residual risk

The classic transcript endpoint still transports the full session history on every poll. This wave avoids reparsing and rerendering unchanged content, but transport remains O(history). A future cursor/replay or compression design must be coordinated with the OpenCode API contract; this PR intentionally does not invent pagination, truncate history, or add a runtime dependency.