fix(update): persist update_channel via settings.json instead of hasGit

After b464fdc3 (preserve .git on release switch), the hasGit-based
channel detection is unreliable — .git now exists for both release
and nightly installs, so page refresh always reset the channel.

- Add _resolveChannelFromSettings() with migration heuristic:
  !hasGit → release (ZIP), detached HEAD → release (on tag),
  on branch → nightly. Uses gitInfo.branch from check-updates.
- Persist resolved channel to settings.json on first load
  (one-time migration) and on explicit switchChannel.
- Add update_channel validation (release|nightly) in backend
  update_settings handler.
- Remove hasGit-based guessing from initialize(); defer to
  checkForUpdates where full gitInfo is available.
- Channel resolution runs before checkForUpdates early-returns
  to avoid null channelMode on reload-within-interval.

Tests: 361 passed.

tests/frontend/managers/updateService.test.js

@@ -1,12 +1,26 @@
 import { describe, beforeEach, afterEach, expect, it, vi } from 'vitest';
 import { UpdateService } from '../../../static/js/managers/UpdateService.js';
+import { state } from '../../../static/js/state/index.js';
 
 function createFetchResponse(payload) {
     return {
-        json: vi.fn().mockResolvedValue(payload)
+        json: vi.fn().mockResolvedValue(payload),
+        ok: true,
     };
 }
 
+function stubSettingsUpdateChannel(channel) {
+    state.global = state.global || {};
+    state.global.settings = state.global.settings || {};
+    state.global.settings.update_channel = channel;
+}
+
+function clearSettingsUpdateChannel() {
+    if (state.global?.settings) {
+        delete state.global.settings.update_channel;
+    }
+}
+
 describe('UpdateService passive checks', () => {
     let service;
     let fetchMock;
@@ -16,10 +30,13 @@ describe('UpdateService passive checks', () => {
             success: true,
             current_version: 'v1.0.0',
             latest_version: 'v1.0.0',
-            git_info: { short_hash: 'abc123' }
+            git_info: { short_hash: 'abc123' },
+            has_git: true,
         }));
         global.fetch = fetchMock;
 
+        stubSettingsUpdateChannel('release');
+
         service = new UpdateService();
         service.updateNotificationsEnabled = false;
         service.lastCheckTime = 0;
@@ -28,6 +45,7 @@ describe('UpdateService passive checks', () => {
 
     afterEach(() => {
         delete global.fetch;
+        clearSettingsUpdateChannel();
     });
 
     it('skips passive update checks when notifications are disabled', async () => {