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

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;

static/index.html

@@ -324,6 +324,13 @@
           <option value="ctrl+enter">Ctrl+Enter (Enter for newline)</option>
         </select>
       </div>
+      <div class="settings-field">
+        <label style="display:flex;align-items:center;gap:8px;cursor:pointer">
+          <input type="checkbox" id="settingsShowTokenUsage" style="width:15px;height:15px;accent-color:var(--accent)">
+          Show token usage after responses
+        </label>
+        <div style="font-size:11px;color:var(--muted);margin-top:4px">Displays input/output token count below each assistant reply. Also toggled with <code>/usage</code>.</div>
+      </div>
       <div class="settings-field" style="border-top:1px solid var(--border);padding-top:12px;margin-top:8px">
         <label for="settingsPassword">Access Password</label>
         <div style="font-size:11px;color:var(--muted);margin-bottom:6px">Enter a new password to set or change it. Leave blank to keep current setting.</div>

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}));

static/panels.js

@@ -958,6 +958,8 @@ async function loadSettingsPanel(){
     // Send key preference
     const sendKeySel=$('settingsSendKey');
     if(sendKeySel) sendKeySel.value=settings.send_key||'enter';
+    const showUsageCb=$('settingsShowTokenUsage');
+    if(showUsageCb) showUsageCb.checked=!!settings.show_token_usage;
     // Password field: always blank (we don't send hash back)
     const pwField=$('settingsPassword');
     if(pwField) pwField.value='';
@@ -979,16 +981,19 @@ async function saveSettings(){
   const model=($('settingsModel')||{}).value;
   const workspace=($('settingsWorkspace')||{}).value;
   const sendKey=($('settingsSendKey')||{}).value;
+  const showTokenUsage=!!($('settingsShowTokenUsage')||{}).checked;
   const pw=($('settingsPassword')||{}).value;
   const body={};
   if(model) body.default_model=model;
   if(workspace) body.default_workspace=workspace;
   if(sendKey) body.send_key=sendKey;
+  body.show_token_usage=showTokenUsage;
   // Password: only act if the field has content; blank = leave auth unchanged
   if(pw && pw.trim()){
     try{
       await api('/api/settings',{method:'POST',body:JSON.stringify({...body,_set_password:pw.trim()})});
       window._sendKey=sendKey||'enter';
+      window._showTokenUsage=showTokenUsage;
       showToast('Settings saved (password set — login now required)');
       toggleSettings();
       return;
@@ -997,6 +1002,8 @@ async function saveSettings(){
   try{
     await api('/api/settings',{method:'POST',body:JSON.stringify(body)});
     window._sendKey=sendKey||'enter';
+    window._showTokenUsage=showTokenUsage;
+    renderMessages();
     showToast('Settings saved');
     toggleSettings();
   }catch(e){

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;}}