feat(web): add config backup & restore UI

Adds a Backup & Restore tab to the v3 web UI that packages user config,
secrets, WiFi, user-uploaded fonts, plugin image uploads, and the installed
plugin list into a single ZIP for safe reinstall recovery. Restore extracts
the bundle, snapshots current state via the existing atomic config manager
(so rollback stays available), reapplies the selected sections, and
optionally reinstalls missing plugins from the store.

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

web_interface/blueprints/pages_v3.py

@@ -76,6 +76,8 @@ def load_partial(partial_name):
             return _load_logs_partial()
         elif partial_name == 'raw-json':
             return _load_raw_json_partial()
+        elif partial_name == 'backup-restore':
+            return _load_backup_restore_partial()
         elif partial_name == 'wifi':
             return _load_wifi_partial()
         elif partial_name == 'cache':
@@ -296,6 +298,13 @@ def _load_raw_json_partial():
     except Exception as e:
         return f"Error: {str(e)}", 500
 
+def _load_backup_restore_partial():
+    """Load backup & restore partial."""
+    try:
+        return render_template('v3/partials/backup_restore.html')
+    except Exception as e:
+        return f"Error: {str(e)}", 500
+
 @pages_v3.route('/setup')
 def captive_setup():
     """Lightweight captive portal setup page — self-contained, no frameworks."""