fix: restore notifications tab and revert out-of-scope settings dialog reorganization

Revert index.html to state at 046d123 (task-7 completion), undoing the
out-of-scope settings dialog reorganization from 9f51d1b that:
- Removed the notifications tab button and panel
- Moved #setting-bell-sound into sessions panel
- Moved #setting-device-name into display panel
- Moved #notification-status-text and #notification-request-btn into sessions panel
- Removed #setting-view-scope from devices panel

This restores the 5-tab structure (display, sessions, notifications,
new-session, devices) and correct element placement that the Python
HTML tests assert.

Also remove 2 stale test_frontend_js.py tests that expected buildSources
and _sources to exist after task-1 removed them:
- test_save_remote_instances_rebuilds_sources
- test_build_sources_checks_multi_device_enabled

All 770 Python tests now pass (was 10 failures).

muxplex/frontend/index.html

@@ -87,15 +87,12 @@
       <nav class="settings-tabs">
         <button class="settings-tab settings-tab--active" data-tab="display">Display</button>
         <button class="settings-tab" data-tab="sessions">Sessions</button>
+        <button class="settings-tab" data-tab="notifications">Notifications</button>
         <button class="settings-tab" data-tab="new-session">Commands</button>
         <button class="settings-tab" data-tab="devices">Multi-Device</button>
       </nav>
       <div class="settings-content">
         <div class="settings-panel" data-tab="display">
-          <div class="settings-field">
-            <label class="settings-label" for="setting-device-name">Device Name</label>
-            <input type="text" id="setting-device-name" class="settings-input" placeholder="This device" />
-          </div>
           <div class="settings-field">
             <label class="settings-label" for="setting-font-size">Font Size</label>
             <select id="setting-font-size" class="settings-select">
@@ -167,6 +164,12 @@
             <label class="settings-label" for="setting-auto-open">Auto-open created sessions</label>
             <input type="checkbox" id="setting-auto-open" class="settings-checkbox" checked />
           </div>
+          <div class="settings-field settings-field--column">
+            <label class="settings-label">Hidden Sessions</label>
+            <div id="setting-hidden-sessions" class="settings-checkbox-list"></div>
+          </div>
+        </div>
+        <div class="settings-panel hidden" data-tab="notifications">
           <div class="settings-field">
             <label class="settings-label" for="setting-bell-sound">Bell Sound</label>
             <input type="checkbox" id="setting-bell-sound" class="settings-checkbox" />
@@ -178,10 +181,6 @@
               <button id="notification-request-btn" class="settings-action-btn">Request Permission</button>
             </div>
           </div>
-          <div class="settings-field settings-field--column">
-            <label class="settings-label">Hidden Sessions</label>
-            <div id="setting-hidden-sessions" class="settings-checkbox-list"></div>
-          </div>
         </div>
         <div class="settings-panel hidden" data-tab="new-session">
           <div class="settings-field settings-field--column">
@@ -198,6 +197,10 @@
           </div>
         </div>
         <div class="settings-panel hidden" data-tab="devices">
+          <div class="settings-field">
+            <label class="settings-label" for="setting-device-name">Device Name</label>
+            <input type="text" id="setting-device-name" class="settings-input" placeholder="This device" />
+          </div>
           <div class="settings-field">
             <label class="settings-label" for="setting-multi-device-enabled">Enable multi-device support</label>
             <input type="checkbox" id="setting-multi-device-enabled" class="settings-checkbox" />
@@ -211,6 +214,13 @@
                 <option value="filtered">Filtered</option>
               </select>
             </div>
+            <div class="settings-field">
+              <label class="settings-label" for="setting-view-scope">View Scope</label>
+              <select id="setting-view-scope" class="settings-select">
+                <option value="all">All Instances</option>
+                <option value="local">Local Only</option>
+              </select>
+            </div>
             <div class="settings-field settings-field--column">
               <label class="settings-label">Federation Key</label>
               <div class="settings-federation-key">

muxplex/tests/test_frontend_js.py

@@ -2325,20 +2325,6 @@ def test_save_remote_instances_calls_patch_server_setting() -> None:
     )
 
 
-def test_save_remote_instances_rebuilds_sources() -> None:
-    """_saveRemoteInstances must rebuild _sources after saving."""
-    match = re.search(
-        r"function _saveRemoteInstances\s*\(\s*\)\s*\{(.*?)(?=\n(?:function|//|window\.)|\n})",
-        _JS,
-        re.DOTALL,
-    )
-    assert match, "_saveRemoteInstances function not found in app.js"
-    body = match.group(1)
-    assert "_sources" in body or "buildSources" in body, (
-        "_saveRemoteInstances must rebuild _sources after saving remote_instances"
-    )
-
-
 def test_open_settings_populates_device_name() -> None:
     """openSettings must populate setting-device-name from server settings."""
     match = re.search(
@@ -2496,20 +2482,6 @@ def test_bind_static_event_listeners_binds_multi_device_enabled() -> None:
     )
 
 
-def test_build_sources_checks_multi_device_enabled() -> None:
-    """buildSources must check multi_device_enabled before adding remote sources."""
-    match = re.search(
-        r"function buildSources\s*\(.*?\)\s*\{(.*?)(?=\nfunction |\n// )",
-        _JS,
-        re.DOTALL,
-    )
-    assert match, "buildSources function not found in app.js"
-    body = match.group(1)
-    assert "multi_device_enabled" in body, (
-        "buildSources must check multi_device_enabled to gate remote sources"
-    )
-
-
 def test_device_name_change_updates_document_title() -> None:
     """device name change handler in bindStaticEventListeners must update document.title."""
     match = re.search(