feat: self-update checker with one-click update for WebUI + Agent

Shows a blue banner when the webui or hermes-agent git repos are behind
their upstream branches. One-click 'Update Now' button does stash, pull
--ff-only, stash pop, then reloads the page.

Backend (api/updates.py):
- _check_repo(): git fetch + rev-list count with 15s timeout
- check_for_updates(): 30-min server-side cache, thread-safe, skips
  Docker (no .git dir)
- apply_update(): stash (if dirty), pull --ff-only, pop, invalidate cache

Routes:
- GET /api/updates/check -- returns cached {webui, agent} with behind count
- POST /api/updates/apply -- {target: 'webui'|'agent'}

Frontend:
- Blue banner (matches reconnect-banner pattern) with 'Later' / 'Update Now'
- Non-blocking boot check via fire-and-forget .then(), once per tab session
- sessionStorage guards prevent re-checking and re-showing after dismiss

Settings:
- 'Check for updates' checkbox (default: on) -- when off, no git operations
- Removed 'Default Workspace' dropdown to keep settings panel compact

Performance:
- Server cache: git fetch at most 2x/hour regardless of client count
- sessionStorage: one check per browser tab session
- _check_in_progress flag prevents concurrent fetch storms
- Fire-and-forget: does NOT block the boot sequence

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/ui.js

@@ -334,6 +334,47 @@ async function refreshSession() {
     showToast('Conversation refreshed');
   } catch(e) { setStatus('Refresh failed: ' + e.message); }
 }
+// ── Update banner ──
+function _showUpdateBanner(data){
+  const parts=[];
+  if(data.webui&&data.webui.behind>0) parts.push(`WebUI: ${data.webui.behind} update${data.webui.behind>1?'s':''}`);
+  if(data.agent&&data.agent.behind>0) parts.push(`Agent: ${data.agent.behind} update${data.agent.behind>1?'s':''}`);
+  if(!parts.length)return;
+  const msg=$('updateMsg');
+  if(msg) msg.textContent='\u2B06 '+parts.join(', ')+' available';
+  const banner=$('updateBanner');
+  if(banner) banner.classList.add('visible');
+  window._updateData=data;
+}
+function dismissUpdate(){
+  const b=$('updateBanner');if(b)b.classList.remove('visible');
+  sessionStorage.setItem('hermes-update-dismissed','1');
+}
+async function applyUpdates(){
+  const btn=$('btnApplyUpdate');
+  if(btn){btn.disabled=true;btn.textContent='Updating\u2026';}
+  const targets=[];
+  if(window._updateData?.webui?.behind>0) targets.push('webui');
+  if(window._updateData?.agent?.behind>0) targets.push('agent');
+  try{
+    for(const target of targets){
+      const res=await api('/api/updates/apply',{method:'POST',body:JSON.stringify({target})});
+      if(!res.ok){
+        showToast('Update failed ('+target+'): '+(res.message||'unknown error'));
+        if(btn){btn.disabled=false;btn.textContent='Update Now';}
+        return;
+      }
+    }
+    showToast('Updated! Reloading\u2026');
+    sessionStorage.removeItem('hermes-update-checked');
+    sessionStorage.removeItem('hermes-update-dismissed');
+    setTimeout(()=>location.reload(),1500);
+  }catch(e){
+    showToast('Update failed: '+e.message);
+    if(btn){btn.disabled=false;btn.textContent='Update Now';}
+  }
+}
+
 async function checkInflightOnBoot(sid) {
   const raw = localStorage.getItem(INFLIGHT_KEY);
   if (!raw) return;