fix(ui): unify extension update check and improve update affordance

- Update availability is now decided solely by the backend live GitHub
  check (updateFlags); the store index may lag and previously masked
  new versions in the market view.
- Replace the ambiguous refresh icon on extension cards with a labeled
  update button, and add a red dot badge on extension icons (card and
  detail view) when an update is available.
- Surface a toast when the update check fails instead of failing silently.

ui/flutter/lib/features/extensions/presentation/widgets/extension_detail_view.dart

@@ -65,12 +65,32 @@ class ExtensionDetailView extends ConsumerWidget {
           key: const ValueKey('extension-details-hero'),
           crossAxisAlignment: CrossAxisAlignment.start,
           children: [
-            ExtensionIcon(
-              item: current,
-              size: mobile ? 60 : 68,
-              borderRadius: BorderRadius.circular(14),
-              fallbackPadding: EdgeInsets.all((mobile ? 60 : 68) * 0.16),
-              fallbackBackgroundColor: palette.surfaceSoft,
+            Stack(
+              clipBehavior: Clip.none,
+              children: [
+                ExtensionIcon(
+                  item: current,
+                  size: mobile ? 60 : 68,
+                  borderRadius: BorderRadius.circular(14),
+                  fallbackPadding: EdgeInsets.all((mobile ? 60 : 68) * 0.16),
+                  fallbackBackgroundColor: palette.surfaceSoft,
+                ),
+                if (canUpdate)
+                  Positioned(
+                    top: -2,
+                    right: -2,
+                    child: Container(
+                      key: const ValueKey('extension-details-update-dot'),
+                      width: 13,
+                      height: 13,
+                      decoration: BoxDecoration(
+                        color: palette.error,
+                        shape: BoxShape.circle,
+                        border: Border.all(color: palette.bg, width: 2.5),
+                      ),
+                    ),
+                  ),
+              ],
             ),
             SizedBox(width: mobile ? 16 : 20),
             Expanded(
@@ -135,7 +155,7 @@ class ExtensionDetailView extends ConsumerWidget {
                 key: const ValueKey('extension-details-update'),
                 loading: false,
                 variant: AppLoadingButtonVariant.primary,
-                icon: const Icon(Icons.refresh, size: 16),
+                icon: const Icon(Icons.upgrade, size: 16),
                 onPressed: busy ? null : () => showExtensionUpdateDialog(context, installed),
                 child: Text(context.l10n.newVersionUpdate),
               ),