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/messages.js

@@ -146,6 +146,9 @@ async function send(){
       }
       if(S.session&&S.session.session_id===activeSid){
         S.session=d.session;S.messages=d.session.messages||[];
+        // Stamp _ts on the last assistant message if it has no timestamp
+        const lastAsst=[...S.messages].reverse().find(m=>m.role==='assistant');
+        if(lastAsst&&!lastAsst._ts&&!lastAsst.timestamp) lastAsst._ts=Date.now()/1000;
         if(d.usage) S.lastUsage=d.usage;
         if(d.session.tool_calls&&d.session.tool_calls.length){
           S.toolCalls=d.session.tool_calls.map(tc=>({...tc,done:true}));