feat(playback): move a session between devices

One device holds the audio; the rest watch the same session over an
event stream and act as remotes. Picking a device hands the audio over
at the position the previous one reported, and that device stops.

Also centre the command palette with margins instead of a translate:
animate-rise sets its own transform and dropped the offset on mobile.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/db/migrations.ts

@@ -819,4 +819,36 @@ export const MIGRATIONS: Migration[] = [
        WHERE t.id = ph.track_id AND ph.track_title IS NULL;
     `,
   },
+  {
+    // One listener, many browsers. `playback_state` is the single authority for
+    // what is playing and which device owns the audio, so a phone can take over
+    // from a desktop mid-track. The queue is stored as whole track objects, not
+    // ids: the device taking over needs to render the queue immediately, and a
+    // snapshot of what was queued at handoff time is the honest thing to move.
+    id: '20260808_playback_devices',
+    sql: `
+      CREATE TABLE IF NOT EXISTS playback_devices (
+        id           UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+        user_id      UUID NOT NULL,
+        name         TEXT NOT NULL,
+        last_seen_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+      );
+      CREATE INDEX IF NOT EXISTS idx_playback_devices_user
+        ON playback_devices (user_id, last_seen_at DESC);
+
+      CREATE TABLE IF NOT EXISTS playback_state (
+        user_id     UUID PRIMARY KEY,
+        device_id   UUID REFERENCES playback_devices(id) ON DELETE SET NULL,
+        track_id    UUID REFERENCES tracks(id) ON DELETE SET NULL,
+        queue       JSONB NOT NULL DEFAULT '[]'::jsonb,
+        queue_index INTEGER NOT NULL DEFAULT -1,
+        position_ms INTEGER NOT NULL DEFAULT 0,
+        is_playing  BOOLEAN NOT NULL DEFAULT FALSE,
+        -- Monotonic per user. A device applies a snapshot only when it is newer
+        -- than the last one it saw, so a delayed delivery cannot rewind anyone.
+        version     BIGINT NOT NULL DEFAULT 0,
+        updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+      );
+    `,
+  },
 ];