Use the real app icon on the login screen

Replace the generic Material cloud icon in a colored circle with the
actual branded app_icon.png (same asset already used for the splash
screen). Checked the rest of the app for other Icons.cloud* usages -
the remaining ones (sync-status chip, activity feed fallback icon)
are functional/semantic, not app branding, so left as-is.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/views/login_view.dart

@@ -145,17 +145,12 @@ class _ServerForm extends StatelessWidget {
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: [
           Center(
-            child: Container(
-              width: 80,
-              height: 80,
-              decoration: BoxDecoration(
-                color: colorScheme.primaryContainer,
-                shape: BoxShape.circle,
-              ),
-              child: Icon(
-                Icons.cloud_outlined,
-                size: 44,
-                color: colorScheme.onPrimaryContainer,
+            child: ClipRRect(
+              borderRadius: BorderRadius.circular(20),
+              child: Image.asset(
+                'assets/icon/app_icon.png',
+                width: 80,
+                height: 80,
               ),
             ),
           ),