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/commands.js

@@ -8,6 +8,7 @@ const COMMANDS=[
   {name:'model',     desc:'Switch model (e.g. /model gpt-4o)',  fn:cmdModel,     arg:'model_name'},
   {name:'workspace', desc:'Switch workspace by name',            fn:cmdWorkspace, arg:'name'},
   {name:'new',       desc:'Start a new chat session',            fn:cmdNew},
+  {name:'usage',     desc:'Toggle token usage display on/off',   fn:cmdUsage},
 ];
 
 function parseCommand(text){
@@ -98,6 +99,19 @@ async function cmdNew(){
   showToast('New session created');
 }
 
+async function cmdUsage(){
+  const next=!window._showTokenUsage;
+  window._showTokenUsage=next;
+  try{
+    await api('/api/settings',{method:'POST',body:JSON.stringify({show_token_usage:next})});
+  }catch(e){}
+  // Update the settings checkbox if the panel is open
+  const cb=$('settingsShowTokenUsage');
+  if(cb) cb.checked=next;
+  renderMessages();
+  showToast('Token usage '+(next?'on':'off'));
+}
+
 // ── Autocomplete dropdown ───────────────────────────────────────────────────
 
 let _cmdSelectedIdx=-1;