Token usage display:
- Add 'show_token_usage' boolean to settings (default: false, off by default)
- Settings panel: checkbox 'Show token usage after responses'
- /usage slash command: instant toggle with toast feedback, persists to
server, updates checkbox if settings panel is open, re-renders messages
- Boot: load show_token_usage alongside send_key on startup
- ui.js: gate usage badge on window._showTokenUsage flag
Timestamps:
- streaming.py: stamp 'timestamp' on every message that lacks one at
conversation completion; old messages (no timestamp field) now get a
wall-clock time the first time they're touched by a new turn
- messages.js: stamp _ts on the last assistant message at done-event time
so the time shows immediately on the current turn before next reload
- Timestamps already render in the UI (Sprint 14): faint time on each
role header line, full opacity on hover, full date in title tooltip
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