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/index.html

@@ -203,6 +203,13 @@
       <div class="messages-inner" id="msgInner"></div>
       <div id="liveToolCards" style="display:none;max-width:800px;margin:0 auto;width:100%;padding:0 24px;"></div>
     </div>
+    <div class="update-banner" id="updateBanner">
+      <span id="updateMsg"></span>
+      <div style="display:flex;gap:8px;flex-shrink:0">
+        <button class="update-btn" onclick="dismissUpdate()">Later</button>
+        <button class="update-btn update-primary" id="btnApplyUpdate" onclick="applyUpdates()">Update Now</button>
+      </div>
+    </div>
     <div class="reconnect-banner" id="reconnectBanner">
       <span id="reconnectMsg">&#9888; A response may have been in progress when you last left. Reload messages?</span>
       <div style="display:flex;gap:8px;flex-shrink:0">
@@ -319,10 +326,6 @@
         <label for="settingsModel">Default Model</label>
         <select id="settingsModel" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px"></select>
       </div>
-      <div class="settings-field">
-        <label for="settingsWorkspace">Default Workspace</label>
-        <select id="settingsWorkspace" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px"></select>
-      </div>
       <div class="settings-field">
         <label for="settingsSendKey">Send Key</label>
         <select id="settingsSendKey" style="width:100%;padding:8px;background:var(--code-bg);color:var(--text);border:1px solid var(--border2);border-radius:6px">
@@ -362,6 +365,13 @@
         </label>
         <div style="font-size:11px;color:var(--muted);margin-top:4px">Mirrors WebUI token usage to state.db so <code>hermes /insights</code> includes browser session data. Off by default.</div>
       </div>
+      <div class="settings-field">
+        <label style="display:flex;align-items:center;gap:8px;cursor:pointer">
+          <input type="checkbox" id="settingsCheckUpdates" style="width:15px;height:15px;accent-color:var(--accent)">
+          Check for updates
+        </label>
+        <div style="font-size:11px;color:var(--muted);margin-top:4px">Show a banner when newer versions of the WebUI or Agent are available. Runs a background git fetch periodically.</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>