Dashboard: hide unreachable servers by default + keep card height consistent

- Unreachable cards now show a disabled 'Starten' button so moderator cards
  stay the same height as reachable ones
- Dashboard hides unreachable servers by default; a toggle button (bottom
  of header area, only visible when unreachable count > 0) flips the view
  and persists the preference in localStorage

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gsm-frontend/src/components/ServerCard.jsx

@@ -288,10 +288,18 @@ export default function ServerCard({ server, onClick, isAuthenticated, isGuest,
       )}
 
       {/* Server Controls - only for moderators */}
-      {isModerator && !isUnreachable && (
+      {isModerator && (
         <div className="mt-3 pt-3 border-t border-neutral-800">
           <div className="flex flex-wrap gap-2">
-            {(server.status === 'online' || (server.running && server.status !== 'starting' && server.status !== 'stopping')) ? (
+            {isUnreachable ? (
+              <button
+                disabled
+                title="Host nicht erreichbar"
+                className="btn btn-primary btn-sm"
+              >
+                Starten
+              </button>
+            ) : (server.status === 'online' || (server.running && server.status !== 'starting' && server.status !== 'stopping')) ? (
               <>
                 <button
                   onClick={(e) => handleActionClick('stop', e)}