feat: buffer/latency control + non-selectable node text

Buffer Control:
- GET /api/quantum - reads current graph quantum via pw-metadata
- POST /api/quantum {quantum:N} - sets quantum via pw-metadata
- Toolbar dropdown with presets: 32, 64, 128, 256, 512, 1024, 2048, 4096
- Loads current quantum on startup

Text Selection Fix:
- Added user-select: none to wrap and canvas CSS
- Node text no longer gets selected when dragging

frontend/src/lib/stores.ts

@@ -441,4 +441,25 @@ export async function unloadModule(moduleId: number) {
   }
 }
 
+// Quantum (buffer size) control
+export async function getQuantum(): Promise<number> {
+  try {
+    const res = await fetch('/api/quantum');
+    const data = await res.json();
+    return data.quantum || 0;
+  } catch { return 0; }
+}
+
+export async function setQuantum(quantum: number) {
+  try {
+    await fetch('/api/quantum', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ quantum }),
+    });
+  } catch (e) {
+    console.error('[api] set-quantum failed:', e);
+  }
+}
+
 export { connectPorts, disconnectPorts };