feat: hide Upload button on play page, use btn-danger for Stop

- Header accepts hideUpload prop, hides the + Upload button when true
- App.svelte passes hideUpload={route === '/play'} to Header
- Play.svelte Stop button now uses global btn-danger class (red, matching
  the Delete button style) instead of custom stop-btn styles

frontend/src/App.svelte

@@ -43,7 +43,7 @@
   });
 </script>
 
-<Header onUploadClick={() => uploadTriggered++} />
+<Header onUploadClick={() => uploadTriggered++} hideUpload={route === "/play"} />
 
 <main class="container">
   {#if route === "/"}

frontend/src/lib/Header.svelte

@@ -1,5 +1,5 @@
 <script>
-  let { onUploadClick = () => {} } = $props();
+  let { onUploadClick = () => {}, hideUpload = false } = $props();
 </script>
 
 <header class="header">
@@ -17,7 +17,9 @@
       <span class="logo-text">DOSTALGIA</span>
     </a>
     <nav>
-      <button class="btn" onclick={onUploadClick}>+ Upload</button>
+      {#if !hideUpload}
+        <button class="btn" onclick={onUploadClick}>+ Upload</button>
+      {/if}
     </nav>
   </div>
 </header>

frontend/src/pages/Play.svelte

@@ -134,7 +134,7 @@
         ← {isSetup ? "Back" : game.title}
       </button>
       {#if running}
-        <button class="btn stop-btn" onclick={stopEmulator}>⏹ Stop</button>
+        <button class="btn btn-danger" onclick={stopEmulator}>⏹ Stop</button>
       {/if}
     {/if}
   </div>
@@ -211,14 +211,6 @@
     font-family: var(--font-sans);
   }
   .link-button:hover { color: var(--phosphor); }
-  .stop-btn {
-    border-color: #cc3333;
-    color: #ff4444;
-  }
-  .stop-btn:hover {
-    background: #331111;
-    box-shadow: 0 0 12px #331111;
-  }
 
   /* Emulator canvas — always mounted */
   .dos-container {