feat(scheduled): add "Send now" action to the scheduled-messages tray

Fire a pending scheduled message immediately via MSC4140 action:'send'
(the server dispatches the stored delayed event now, as a normal timeline
event) instead of having to cancel and retype.

- sendScheduledMessageNow(mx, delayId) mirrors cancel/restart with action:'send'
- handleSendNow reuses the per-row busy guard; prunes local state only once the
  server confirms; a failed send shows an inline "Could not send now" error with
  the message still sendable/editable/cancellable
- Send-now IconButton (Icons.Send) added before Edit/Cancel in each row

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/utils/scheduledMessages.ts

@@ -53,3 +53,19 @@ export async function restartScheduledMessage(mx: MatrixClient, delayId: string)
     { prefix: '/_matrix/client/unstable/org.matrix.msc4140' },
   );
 }
+
+/**
+ * Send a scheduled message immediately via MSC4140 (`action: 'send'`) — the
+ * server dispatches the stored delayed event now, as a normal timeline event,
+ * instead of waiting for its timeout.
+ */
+export async function sendScheduledMessageNow(mx: MatrixClient, delayId: string): Promise<void> {
+  const path = `/delayed_events/${encodeURIComponent(delayId)}`;
+  await mx.http.authedRequest(
+    Method.Post,
+    path,
+    undefined,
+    { action: 'send' },
+    { prefix: '/_matrix/client/unstable/org.matrix.msc4140' },
+  );
+}