feat: last-message preview, idle indicator, and rename UX polish

[researchoor]

Summary

• Last-message preview: cache last assistant message per session (UserDefaults) and show as one-line preview on session list — same pattern as iMessage. Populated when a thread is opened; persists across app restarts.
• Idle indicator: show muted dot on completed session rows instead of invisible spacer.
• Rename UX: pre-fill rename dialog with current session title for easier editing.
• Live Activity guard: prevent background wake from updating Live Activity for already-completed sessions.

Reworked per feedback — dropped the local-only nicknames approach. Rename uses the existing thread/name/set server-side API (already wired up via ServerManager.renameThread), so names sync across devices.

Changed files

File	Change
LocalSessionLastMessage.swift	New: on-device cache for last assistant message per session
ServerManager.swift	Guard Live Activity updates for completed sessions
SessionsModel.swift	Orchestrate last-message cache updates outside observation tracking
SessionsScreen.swift	Last-message preview row, idle dot, pre-filled rename draft
project.pbxproj	Add new file to project

Known limitation

Last-message preview requires opening a thread at least once to populate the cache. Once openai/codex#15350 ships last_assistant_message in thread/list, the cache can be replaced with the server field.

Verification

1. Build: xcodebuild -project apps/ios/Litter.xcodeproj -scheme Litter -configuration Debug -destination 'platform=iOS Simulator,name=iPhone 17 Pro' build
2. Open a session → go back to list → verify last-message preview appears
3. Long-press session → Rename → verify title pre-filled and rename persists server-side
4. Completed sessions show muted idle dot

🤖 Generated with Claude Code

[dnakov]

i think we should just be using the app-server way to rename the thread, no?