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/ui.js

@@ -488,8 +488,8 @@ function renderMessages(){
       else inner.appendChild(frag);
     }
   }
-  // Render usage badge on the last assistant message row (if usage data exists)
-  if(S.session&&(S.session.input_tokens||S.session.output_tokens)){
+  // Render usage badge on the last assistant message row (if enabled and usage data exists)
+  if(window._showTokenUsage&&S.session&&(S.session.input_tokens||S.session.output_tokens)){
     const rows=inner.querySelectorAll('.msg-row');
     let lastAssist=null;
     for(let i=rows.length-1;i>=0;i--){if(rows[i].dataset.role==='assistant'){lastAssist=rows[i];break;}}