feat(DPoP): UI: bcast events to support multi tab

add also playwright test with the multi tab

frontend/src/lib/auth/session-broadcast.ts

@@ -0,0 +1,78 @@
+/**
+ * Cross-tab session invalidation via `BroadcastChannel` — the
+ * "logout on tab A, tab B knows immediately" wire.
+ *
+ * Why this exists: after a logout on Tab A, Tab B still holds an
+ * in-memory `CryptoKey` handle to the (now-cleared) DPoP keypair
+ * and a session cookie whose server-side row was just revoked.
+ * Without a cross-tab signal, Tab B doesn't notice until its next
+ * network request — at which point the server 401s and the SPA
+ * bounces to `/login`. That's correctness-safe (see
+ * `docs/plan/dpop.md` Gate 8), but UX-poor: an idle Tab B silently
+ * pretends to be logged in for as long as it stays idle.
+ *
+ * This module fires a `BroadcastChannel` message so every other
+ * tab of the same origin can react synchronously — reset its
+ * session store, redirect to `/login`, no visible drift.
+ *
+ * Scope: session **invalidation** only. Not for cross-tab login
+ * (a fresh sign-in on Tab B while Tab A sits on `/login`); that's
+ * a general auth-store consistency concern, not DPoP-specific,
+ * and can be layered on later using the same primitive if needed.
+ *
+ * Fail-open contract mirrors the rest of the DPoP stack: if
+ * `BroadcastChannel` is unavailable (very old Safari, restricted
+ * webviews), broadcast/subscribe are no-ops. Users lose the
+ * instant-redirect UX; the natural 401-on-next-request path
+ * kicks in as before.
+ */
+
+const CHANNEL_NAME = 'oxicloud-session-cleared';
+
+/**
+ * Post a "session cleared" event to every other tab of this
+ * origin. The current tab does NOT receive its own message —
+ * `BroadcastChannel` skips the sender by design.
+ *
+ * Called from `logout()` after the server round trip completes
+ * (success or failure — user intent is what matters). Non-fatal
+ * on failure so the logout flow always finishes.
+ */
+export function broadcastSessionCleared(): void {
+	try {
+		const ch = new BroadcastChannel(CHANNEL_NAME);
+		ch.postMessage({ kind: 'session_cleared', at: Date.now() });
+		ch.close();
+	} catch (err) {
+		console.debug('session-broadcast: postMessage failed', err);
+	}
+}
+
+/**
+ * Subscribe to cross-tab session-cleared events. Wire this once
+ * from the root layout's `onMount`; the callback should reset
+ * the SPA's session store and navigate to `/login`.
+ *
+ * Returns a cleanup function that closes the channel — call it
+ * from the layout's `onDestroy` so hot-reload during dev doesn't
+ * leak listeners.
+ *
+ * Errors during subscription are swallowed to a no-op: same
+ * degradation posture as the rest of the DPoP stack.
+ */
+export function onSessionCleared(callback: () => void): () => void {
+	try {
+		const ch = new BroadcastChannel(CHANNEL_NAME);
+		ch.onmessage = () => {
+			try {
+				callback();
+			} catch (err) {
+				console.debug('session-broadcast: callback threw', err);
+			}
+		};
+		return () => ch.close();
+	} catch (err) {
+		console.debug('session-broadcast: subscribe failed', err);
+		return () => {};
+	}
+}