fix: prevent js-dos CSS from overriding custom button styles

The Play page injects js-dos CSS from CDN which has its own .btn
rules. Because it loads after the app CSS, it was overriding the
custom phosphor-green button styles (including the Upload button
in the header).

Fixed by doubling the class selector (.btn.btn) to raise specificity
to (0,2,0), which beats injected CSS regardless of load order.

frontend/src/app.css

@@ -98,7 +98,8 @@ h3 { font-size: 1.2rem; }
 /* ═══════════════════════════════════════════════════════════════
    BUTTONS
    ═══════════════════════════════════════════════════════════════ */
-.btn {
+/* Double-class selector to win specificity against injected CSS */
+.btn.btn {
   display: inline-flex;
   align-items: center;
   gap: 8px;
@@ -113,30 +114,30 @@ h3 { font-size: 1.2rem; }
   cursor: pointer;
   transition: all 0.15s ease;
 }
-.btn:hover {
+.btn.btn:hover {
   background: var(--phosphor-burn);
   border-color: var(--phosphor);
   box-shadow: 0 0 12px var(--phosphor-dark);
 }
-.btn:active {
+.btn.btn:active {
   transform: scale(0.97);
 }
 
-.btn-primary {
+.btn-primary.btn-primary {
   background: var(--phosphor-dark);
   border-color: var(--phosphor);
   color: var(--phosphor-glow);
 }
-.btn-primary:hover {
+.btn-primary.btn-primary:hover {
   background: var(--phosphor-dim);
   color: var(--bg);
 }
 
-.btn-danger {
+.btn-danger.btn-danger {
   border-color: #cc3333;
   color: #cc3333;
 }
-.btn-danger:hover {
+.btn-danger.btn-danger:hover {
   background: #331111;
   border-color: #ff4444;
   color: #ff4444;
@@ -187,7 +188,7 @@ input::placeholder {
 @media (max-width: 640px) {
   h1 { font-size: 1.5rem; }
   h2 { font-size: 1.25rem; }
-  .btn { padding: 8px 14px; font-size: 0.85rem; min-height: 40px; }
+  .btn.btn { padding: 8px 14px; font-size: 0.85rem; min-height: 40px; }
   input, textarea, select { font-size: 16px; } /* prevents iOS zoom */
 }