Fix: Restore Native caching business logic and implement 3-mode launcher

- Implemented Default, Onsite, and Native launcher modes in launcher_file_cont.svelte.
- Restored background pre-caching logic with configurable timers in new LauncherBackgroundSync component.
- Fixed standard browser download regression for regular website mode.
- Modernized electron_relay to TypeScript and standardized bridge detection in layout.
- Detailed startup and background sync technical flow in documentation.

src/routes/+layout.svelte

@@ -645,11 +645,12 @@
         }
 
         // *** Electron Native Mode Detection ***
-        // If window.native_app exists, we are running inside the Electron bridge
-        // @ts-ignore - native_app is injected by the Electron preload script
-        if (window.native_app) {
+        // If window.native_app or window.aetherNative exists, we are running inside the Electron bridge
+        // @ts-ignore - native_app is injected by legacy, aetherNative by new V3 bridge
+        if (window.native_app || window.aetherNative) {
             console.log('ELECTRON: Native environment detected. Switching to native app_mode.');
             $events_loc.launcher.app_mode = 'native';
+            $ae_loc.is_native = true;
         }
     }