feat: add multi-user support for favorites management and room clock synchronization

- Implemented a new API endpoint for retrieving and managing user favorites in /api/users.
- Added functionality for admins to edit the shared "main" user's favorites.
- Created a one-shot DB smoke test script for verifying multi-user kiosk migrations.
- Introduced a RoomClock class for synchronizing server time across clients using WebSocket.

electron/preload.cjs

@@ -0,0 +1,107 @@
+// Preload for the Master Electron window.
+//
+// Exposes `window.oradioNative` to the renderer via `contextBridge`. This is
+// the single source of truth for "are we running inside Electron?" — the
+// master UI uses `oradioNative?.isElectron` to gate features that don't work
+// in a plain browser tab (real audio-output enumeration/selection, the live
+// spectrum visualizer's cross-origin PCM tap, etc.).
+//
+// Runs in a sandboxed isolated world (sandbox: true, contextIsolation: true),
+// so it has access to a limited subset of Electron APIs and to the renderer's
+// `navigator.mediaDevices`. No Node built-ins.
+//
+// Written in CommonJS (`.cjs`) on purpose: the repo's package.json sets
+// `"type": "module"`, and Electron's preload loader expects CJS.
+
+const { contextBridge } = require('electron');
+
+// ---- Audio output enumeration / selection ------------------------------
+
+// Classify a device label into a coarse `kind` so the master UI can show an
+// appropriate icon. Falls back to 'speakers' for the system default.
+function classifyKind(label) {
+    const s = String(label || '').toLowerCase();
+    if (/bluetooth|\bbt\b|airpods|buds|stanmore|bose|sonos/.test(s)) return 'bluetooth';
+    if (/hdmi|display|tv|monitor/.test(s)) return 'hdmi';
+    if (/usb|audient|focusrite|scarlett|presonus|motu/.test(s)) return 'usb';
+    if (/headphone|headset|earphone|jack/.test(s)) return 'headphones';
+    return 'speakers';
+}
+
+let labelsUnlocked = false;
+const currentListeners = new Set();
+let lastCurrentId = 'default';
+
+async function unlockDeviceLabels() {
+    if (labelsUnlocked) return;
+    // Asking for an audio input stream gives Chromium permission to expose
+    // device LABELS in subsequent enumerateDevices() calls. The Electron main
+    // process auto-grants the `media` permission so this is silent.
+    try {
+        const stream = await navigator.mediaDevices.getUserMedia({ audio: true });
+        for (const t of stream.getTracks()) t.stop();
+        labelsUnlocked = true;
+    } catch (err) {
+        // Permission denied or no input device — labels will be empty but we
+        // can still surface the IDs (and the system default).
+        console.warn('[preload] could not unlock device labels:', err?.message || err);
+    }
+}
+
+async function listOutputs() {
+    if (!navigator.mediaDevices?.enumerateDevices) return [];
+    await unlockDeviceLabels();
+    const all = await navigator.mediaDevices.enumerateDevices();
+    const out = all
+        .filter((d) => d.kind === 'audiooutput')
+        .map((d, i) => ({
+            id: d.deviceId,
+            label: d.label || (d.deviceId === 'default' ? 'System default' : `Audio output ${i + 1}`),
+            kind: classifyKind(d.label)
+        }));
+    // Ensure 'default' is always first when present.
+    out.sort((a, b) => (a.id === 'default' ? -1 : b.id === 'default' ? 1 : 0));
+    return out;
+}
+
+async function setOutput(id) {
+    lastCurrentId = String(id || 'default');
+    // The actual `audio.setSinkId(id)` call has to happen on the renderer's
+    // <audio> element, which lives in the master's main-world context. The
+    // master code calls setSinkId directly after this resolves — preload only
+    // tracks the selection and notifies listeners.
+    for (const cb of currentListeners) {
+        try { cb(lastCurrentId); } catch { /* ignore listener errors */ }
+    }
+    return lastCurrentId;
+}
+
+function getCurrent() {
+    return lastCurrentId;
+}
+
+function onCurrentChanged(cb) {
+    if (typeof cb !== 'function') return () => { };
+    currentListeners.add(cb);
+    return () => currentListeners.delete(cb);
+}
+
+// Re-notify when the OS device set changes (cable plug/unplug, Bluetooth
+// connect/disconnect). The renderer can call listOutputs() again on this.
+if (navigator.mediaDevices?.addEventListener) {
+    navigator.mediaDevices.addEventListener('devicechange', () => {
+        for (const cb of currentListeners) {
+            try { cb(lastCurrentId); } catch { /* ignore */ }
+        }
+    });
+}
+
+// ---- Bridge ------------------------------------------------------------
+
+contextBridge.exposeInMainWorld('oradioNative', {
+    isElectron: true,
+    listOutputs,
+    setOutput,
+    getCurrent,
+    onCurrentChanged
+});