add shared computer feature

frontend/src/lib/client/shared-device.ts

@@ -0,0 +1,32 @@
+// Per-device "this is a shared computer" flag. localStorage is the source of
+// truth (shared-ness is a property of THIS browser, not the family — a DB
+// flag would force PIN mode on every device including a parent's phone).
+// A plain cookie mirror lets server loads see it too (localStorage never
+// reaches the server). Neither is a security boundary: the PIN + device
+// session cookies remain the actual gate (see shared-device.md).
+const LS_KEY = 'fam_shared_device';
+const COOKIE = 'fam_shared_device';
+
+export function isSharedDevice(): boolean {
+	if (typeof localStorage === 'undefined') return false;
+	try {
+		return localStorage.getItem(LS_KEY) === '1';
+	} catch {
+		return false;
+	}
+}
+
+export function setSharedDevice(on: boolean) {
+	try {
+		if (on) localStorage.setItem(LS_KEY, '1');
+		else localStorage.removeItem(LS_KEY);
+	} catch {
+		/* private mode etc. — flag simply doesn't persist */
+	}
+	if (typeof document !== 'undefined') {
+		document.cookie =
+			on
+				? `${COOKIE}=1; path=/; max-age=31536000; SameSite=Lax`
+				: `${COOKIE}=; path=/; max-age=0; SameSite=Lax`;
+	}
+}