feat: notification sound and browser notifications on task completion

Add two new settings (both default off):
- sound_enabled: plays a short tone via Web Audio API when assistant
  finishes a response or requests approval
- notifications_enabled: shows a browser notification when a response
  completes while the tab is in the background

Uses Web Audio API (oscillator) instead of bundled MP3 file — zero
additional assets. Follows the standard 4-file settings pattern.

Also skip test_valid_skill_accepted when hermes-agent not installed
(skills endpoint returns 500 without the agent module).

Inspired by #176 (DavidSchuchert)

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

static/index.html

@@ -345,6 +345,20 @@
           <option value="oled">OLED</option>
         </select>
       </div>
+      <div class="settings-field">
+        <label style="display:flex;align-items:center;gap:8px;cursor:pointer">
+          <input type="checkbox" id="settingsSoundEnabled" style="width:15px;height:15px;accent-color:var(--accent)">
+          Notification sound
+        </label>
+        <div style="font-size:11px;color:var(--muted);margin-top:4px">Play a sound when the assistant finishes a response.</div>
+      </div>
+      <div class="settings-field">
+        <label style="display:flex;align-items:center;gap:8px;cursor:pointer">
+          <input type="checkbox" id="settingsNotificationsEnabled" style="width:15px;height:15px;accent-color:var(--accent)">
+          Browser notifications
+        </label>
+        <div style="font-size:11px;color:var(--muted);margin-top:4px">Show a system notification when a response completes while the tab is in the background.</div>
+      </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)">