v2.2.1: Auto-install build tools (aapt2/ecj/d8/apksigner), dev tools banner with one-tap install

www/css/styles.css

@@ -1107,6 +1107,51 @@ a:hover { text-decoration: underline; }
     -webkit-overflow-scrolling: touch;
 }
 
+/* Dev Tools Banner */
+.dev-tools-banner {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 10px 14px;
+    margin: 8px 12px;
+    background: rgba(255, 165, 2, 0.1);
+    border: 1px solid var(--warning);
+    border-radius: var(--radius-sm);
+    font-size: 13px;
+    color: var(--text-primary);
+    animation: fadeIn 0.3s ease;
+}
+.dtb-icon {
+    font-size: 16px;
+    flex-shrink: 0;
+}
+.dtb-msg {
+    flex: 1;
+    line-height: 1.4;
+}
+.dtb-install-btn {
+    background: var(--accent);
+    color: white;
+    border: none;
+    padding: 6px 14px;
+    border-radius: 6px;
+    font-size: 12px;
+    font-weight: 700;
+    cursor: pointer;
+    flex-shrink: 0;
+    white-space: nowrap;
+}
+.dtb-install-btn:disabled { opacity: 0.6; cursor: not-allowed; }
+.dtb-dismiss-btn {
+    background: none;
+    border: none;
+    color: var(--text-muted);
+    font-size: 18px;
+    cursor: pointer;
+    padding: 0 4px;
+    flex-shrink: 0;
+}
+
 /* Responsive */
 @media (max-width: 480px) {
     .message { max-width: 92%; }