feat: token usage toggle (setting + /usage command) + timestamp fixes

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

static/boot.js

@@ -308,7 +308,7 @@ document.querySelectorAll('.suggestion').forEach(btn=>{
 
 (async()=>{
   // Load send key preference
-  try{const s=await api('/api/settings');window._sendKey=s.send_key||'enter';}catch(e){window._sendKey='enter';}
+  try{const s=await api('/api/settings');window._sendKey=s.send_key||'enter';window._showTokenUsage=!!s.show_token_usage;}catch(e){window._sendKey='enter';window._showTokenUsage=false;}
   // Fetch active profile
   try{const p=await api('/api/profile/active');S.activeProfile=p.name||'default';}catch(e){S.activeProfile='default';}
   // Update profile chip label immediately