fix(calls): make leaving a call final and clean up stranded calls

apps/web/src/features/channel/Call/CallAlonePrompt.tsx

@@ -0,0 +1,45 @@
+import { ConfirmDialog } from '@ui/components/ConfirmDialog';
+import { createSignal, onCleanup } from 'solid-js';
+
+function formatRemaining(ms: number) {
+  const totalSeconds = Math.max(0, Math.ceil(ms / 1000));
+  const minutes = Math.floor(totalSeconds / 60);
+  const seconds = totalSeconds % 60;
+  return `${minutes}:${seconds.toString().padStart(2, '0')}`;
+}
+
+/**
+ * Asks a user who is alone in a call whether to keep it going before the
+ * server ends it at `endsAt`. Dismissing leaves the deadline in place.
+ */
+export function CallAlonePrompt(props: {
+  /** Epoch ms when the server ends the call. */
+  endsAt: number;
+  pending: boolean;
+  onStay: () => void;
+  onDismiss: () => void;
+}) {
+  const [nowMs, setNowMs] = createSignal(Date.now());
+  const countdownTimer = globalThis.setInterval(
+    () => setNowMs(Date.now()),
+    1000
+  );
+  onCleanup(() => globalThis.clearInterval(countdownTimer));
+
+  return (
+    <ConfirmDialog
+      open
+      onOpenChange={(open) => {
+        if (!open) props.onDismiss();
+      }}
+      title="You're the only one here"
+      body={`No one else is in this call. It will end in ${formatRemaining(
+        props.endsAt - nowMs()
+      )} unless you stay.`}
+      confirmLabel="Stay in call"
+      cancelLabel="Dismiss"
+      pending={props.pending}
+      onConfirm={props.onStay}
+    />
+  );
+}