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/pages/Play.svelte

@@ -12,6 +12,7 @@
   let error = $state("");
   let booting = $state(false);
   let running = $state(false);
+  let unsupported = $state(false);
   let started = false;
 
   let dosContainer;
@@ -23,6 +24,10 @@
     isSetup = window.location.hash.includes("setup=1");
     try {
       game = await getGame(id);
+      // Windows games can't run in pure DOSBox — flag it
+      if (game.platform === 'windows') {
+        unsupported = true;
+      }
     } catch (e) {
       error = "Game not found";
     }
@@ -76,6 +81,11 @@
     window.location.reload();
   }
 
+  function handleTryAnyway() {
+    unsupported = false;
+    startEmulator();
+  }
+
   function goBack() {
     push(`/game/${id}`);
   }
@@ -84,7 +94,7 @@
 
   // Auto-start when both game data and the container exist
   $effect(() => {
-    if (game?.ready && dosContainer && !started) {
+    if (game?.ready && dosContainer && !started && !unsupported) {
       startEmulator();
     }
   });
@@ -121,6 +131,19 @@
     <div class="error-box">{error}</div>
   {/if}
 
+  <!-- Unsupported platform warning -->
+  {#if unsupported}
+    <div class="overlay">
+      <div class="overlay-icon">🪟</div>
+      <h2>Windows game</h2>
+      <p><strong>{game.title}</strong> is a Windows application and won't run in the browser DOS emulator without Windows 3.1 installed.</p>
+      <div class="unsupported-actions">
+        <button class="btn btn-secondary" onclick={goBack}>← Back</button>
+        <button class="btn" onclick={handleTryAnyway}>Try anyway</button>
+      </div>
+    </div>
+  {/if}
+
   <!-- Loading / booting overlay (covers the canvas while loading) -->
   {#if !running}
     <div class="overlay">
@@ -256,4 +279,9 @@
     border-radius: var(--radius);
     z-index: 10;
   }
+  .unsupported-actions {
+    display: flex;
+    gap: 12px;
+    margin-top: 20px;
+  }
 </style>