fix: persist chat responses to survive navigation

[rogerchappel]

Summary

• Fixes MC fix: tune iOS VAD thresholds to reduce keyboard noise sensitivity #401 (CRITICAL): Agent responses were lost when navigating away from chat during streaming
• Streaming content is now tracked via refs and persisted to DB + localStorage on component unmount
• Message loading on mount now merges all sources (gateway + DB + localStorage) instead of short-circuiting, ensuring persisted messages are never missed
• Cancelled (Escape) responses are also persisted to DB

Root cause

Streaming responses lived only in React component state (streamingContent). Navigating away unmounted the component, and the post-stream persistence code (persistMessage) never ran because the while(true) reader loop was abandoned mid-flight.

What changed

Single file: src/app/chat/page.tsx

1. Ref-based streaming tracking — streamingContentRef and streamingAgentRef mirror the streaming state so cleanup effects can read them
2. Unmount persistence — A cleanup effect fires on unmount: if streaming was in progress, it persists the partial response (marked _(interrupted)_) to both DB and localStorage
3. Merged message loading — On mount, messages are now loaded from all three sources (gateway, DB, localStorage), deduplicated by ID, and sorted by timestamp. Previously, gateway results caused DB to be skipped entirely
4. Cancelled response persistence — Escape-cancelled responses now also persist to DB via persistMessage

Test plan

• Start a chat with an agent, send a message that triggers a long response
• While agent is streaming, navigate to /tasks
• Navigate back to /chat — the partial response should appear with _(interrupted)_ suffix
• Send a message, let it complete fully, navigate away and back — full response preserved
• Press Escape during streaming — cancelled response persists across navigation
• Verify typecheck passes (pnpm typecheck)

Closes MC #401

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

[rogerchappel]

Sentinel — LGTM ✓

Solid approach: refs track in-flight streaming content, and the cleanup effect persists partial responses on unmount (navigation away). The three-way merge (gateway + DB + localStorage) with dedup is a good improvement over the previous cascade fallback.

Minor observations:

• The _(interrupted)_ and _(cancelled)_ suffixes are a nice UX touch — makes it clear the response was incomplete.
• The useEffect cleanup that calls persistMessage on unmount fires a network request during teardown. If the component remounts quickly (React strict mode in dev), you could get duplicate persists. In practice this is deduplicated by the merge logic, so not a real issue, just worth knowing.
• The empty catch {} block in the cleanup localStorage write is fine — best-effort is correct there.

No blocking issues.