feat: show running PulseAudio apps in a dropdown for easier knob targeting

- Add GET /api/apps endpoint that queries pulsectl for active sink inputs
  and returns deduplicated app names (application.name + process.binary)
- Add fetchRunningApps() API helper (returns [] on any failure, non-fatal)
- Fetch running apps on page load; expose a Refresh button in the Knobs
  section header to re-query without a full page reload
- KnobCard app_volume: wire target input to an HTML5 <datalist> so the
  browser shows a native autocomplete dropdown of running apps
- KnobCard group_volume: add a select + Add button beneath the textarea so
  users can pick a running app and append it without typing
- CSS: style the app-picker row and the compact refresh button in h2

src/turnup/ui/server.py

@@ -114,6 +114,30 @@ async def save_config(request: Request) -> dict[str, bool]:
     return {"ok": True}
 
 
+# ── Running apps API ──────────────────────────────────────────────────────────
+
+@app.get("/api/apps")
+def list_running_apps() -> list[str]:
+    """Return unique app names from currently active PulseAudio/PipeWire sink inputs.
+
+    Returns both ``application.name`` and ``application.process.binary`` values
+    (deduplicated) because the daemon's matching logic accepts either.  Falls
+    back to an empty list if pulsectl is unavailable or no server is reachable.
+    """
+    try:
+        import pulsectl  # optional — only present when [ui] extra is installed
+        with pulsectl.Pulse("turnup-ui-apps") as pulse:
+            seen: set[str] = set()
+            for si in pulse.sink_input_list():
+                for field in ("application.name", "application.process.binary"):
+                    val = (si.proplist.get(field) or "").strip()
+                    if val:
+                        seen.add(val)
+            return sorted(seen, key=str.lower)
+    except Exception:
+        return []
+
+
 # ── Presets API ────────────────────────────────────────────────────────────────
 
 def _preset_path(name: str) -> Path:

ui/src/App.tsx

@@ -20,6 +20,8 @@ export default function App() {
   const [toasts,      setToasts]      = useState<ToastItem[]>([]);
   const [loading,     setLoading]     = useState(true);
   const [saving,      setSaving]      = useState(false);
+  const [runningApps, setRunningApps] = useState<string[]>([]);
+  const [appsLoading, setAppsLoading] = useState(false);
 
   const addToast = useCallback((message: string, type: ToastItem['type'] = 'success') => {
     const id = ++toastSeq;
@@ -39,6 +41,15 @@ export default function App() {
     }
   }, []);
 
+  const loadApps = useCallback(async () => {
+    setAppsLoading(true);
+    try {
+      setRunningApps(await api.fetchRunningApps());
+    } finally {
+      setAppsLoading(false);
+    }
+  }, []);
+
   // Initial load
   useEffect(() => {
     void (async () => {
@@ -52,8 +63,9 @@ export default function App() {
         setLoading(false);
       }
       await loadPresets();
+      await loadApps();
     })();
-  }, [addToast, loadPresets]);
+  }, [addToast, loadPresets, loadApps]);
 
   const isDirty = config !== null && JSON.stringify(config) !== JSON.stringify(saved);
 
@@ -128,13 +140,24 @@ export default function App() {
         />
 
         <section className="card" id="knobs">
-          <h2>Knobs</h2>
+          <h2>
+            Knobs
+            <button
+              className="btn-secondary btn-refresh-apps"
+              onClick={() => void loadApps()}
+              disabled={appsLoading}
+              title="Refresh list of running apps"
+            >
+              {appsLoading ? '…' : '↻'} Running apps
+            </button>
+          </h2>
           <div className="cards-grid">
             {KNOB_INDICES.map((i) => (
               <KnobCard
                 key={i}
                 index={Number(i)}
                 knob={config.knobs[i] ?? { action: 'sink_volume', target: 'default' }}
+                runningApps={runningApps}
                 onChange={(knob) =>
                   patchConfig('knobs', { ...config.knobs, [i]: knob })
                 }

ui/src/api.ts

@@ -56,3 +56,13 @@ export async function deletePreset(name: string): Promise<void> {
   });
   await checkResponse(r);
 }
+
+export async function fetchRunningApps(): Promise<string[]> {
+  try {
+    const r = await fetch('/api/apps');
+    if (!r.ok) return [];
+    return r.json();
+  } catch {
+    return [];
+  }
+}

ui/src/components/KnobCard.tsx

@@ -18,11 +18,13 @@ const DEFAULT_LED: LedConfig = {
 interface Props {
   index:        number;
   knob:         KnobConfig;
+  runningApps?: string[];
   onChange:     (knob: KnobConfig) => void;
 }
 
-export function KnobCard({ index, knob, onChange }: Props) {
+export function KnobCard({ index, knob, runningApps = [], onChange }: Props) {
   const [ledOpen,      setLedOpen]      = useState(!!knob.led);
+  const [pickedApp,    setPickedApp]    = useState('');
 
   const update = (patch: Partial<KnobConfig>) => onChange({ ...knob, ...patch });
 
@@ -45,6 +47,19 @@ export function KnobCard({ index, knob, onChange }: Props) {
     }
   };
 
+  const datalistId = `knob-${index}-apps`;
+
+  // Add the selected running app to the group_volume targets list
+  const addPickedApp = () => {
+    const app = pickedApp.trim();
+    if (!app) return;
+    const current = knob.targets ?? [];
+    if (!current.includes(app)) {
+      update({ targets: [...current, app] });
+    }
+    setPickedApp('');
+  };
+
   return (
     <div className="knob-card">
       <div className="card-title">
@@ -87,6 +102,28 @@ export function KnobCard({ index, knob, onChange }: Props) {
               })
             }
           />
+          {runningApps.length > 0 && (
+            <div className="app-picker">
+              <select
+                value={pickedApp}
+                onChange={(e) => setPickedApp(e.target.value)}
+                aria-label="Pick a running app"
+              >
+                <option value="">— running apps —</option>
+                {runningApps.map((app) => (
+                  <option key={app} value={app}>{app}</option>
+                ))}
+              </select>
+              <button
+                type="button"
+                className="btn-secondary"
+                onClick={addPickedApp}
+                disabled={!pickedApp}
+              >
+                Add
+              </button>
+            </div>
+          )}
         </div>
       ) : (
         <div>
@@ -94,10 +131,18 @@ export function KnobCard({ index, knob, onChange }: Props) {
           <input
             type="text"
             id={`knob-${index}-target`}
+            list={runningApps.length > 0 ? datalistId : undefined}
             value={knob.target ?? 'default'}
             placeholder="default"
             onChange={(e) => update({ target: e.target.value })}
           />
+          {runningApps.length > 0 && (
+            <datalist id={datalistId}>
+              {runningApps.map((app) => (
+                <option key={app} value={app} />
+              ))}
+            </datalist>
+          )}
         </div>
       )}
 

ui/src/index.css

@@ -354,3 +354,35 @@ main {
   main { padding: 16px; }
   #app-header { padding: 10px 16px; }
 }
+
+/* ── Running-apps refresh button (Knobs section heading) ───────────────────── */
+#knobs h2 {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+}
+
+.btn-refresh-apps {
+  padding: 3px 10px;
+  font-size: 11px;
+  font-weight: 500;
+  letter-spacing: normal;
+  text-transform: none;
+}
+
+/* ── App picker (group_volume) ─────────────────────────────────────────────── */
+.app-picker {
+  display: flex;
+  gap: 6px;
+  margin-top: 6px;
+}
+
+.app-picker select {
+  flex: 1;
+  min-width: 0;
+}
+
+.app-picker .btn-secondary {
+  flex-shrink: 0;
+  padding: 7px 12px;
+}