Add platform auto-detection and UI flagging for Windows games

- Game.java: add 'platform' field ('dos'/'windows') + 'isPlayable' helper
- UploadResource.java: detect EXE type by reading PE/NE header during upload
- GameResource.java: allow PATCH to update platform
- GameCard.svelte: show 🪟 Win badge for Windows games (blue style)
- GameDetail.svelte: show platform badge, disable Play for Windows with
  'Unplayable' button; add platform dropdown to edit form
- Play.svelte: show blocking overlay for Windows games with 'Try anyway'
  option for users who want to attempt it

frontend/src/lib/GameCard.svelte

@@ -29,7 +29,13 @@
     {/if}
   </div>
   {#if game.ready}
-    <div class="badge">Ready</div>
+    <div class="badge" class:badge-windows={game.platform === 'windows'}>
+      {#if game.platform === 'windows'}
+        🪟 Win
+      {:else}
+        Ready
+      {/if}
+    </div>
   {/if}
 </button>
 
@@ -111,4 +117,8 @@
     font-weight: 600;
     letter-spacing: 0.05em;
   }
+  .badge-windows {
+    background: #1a3a5c;
+    color: #6ab0ff;
+  }
 </style>