fix(maestro): collapse mobile drawer after spawning a session

mountSpawned now fires a CLOSE_DRAWER_EVENT so the sidebar gets out of
the way once the new session lands. Covers fast-forward, modal, and
command-bar spawn paths.

Closes #261

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/uiEvents.ts

@@ -57,6 +57,15 @@ export function requestToggleDrawer(): void {
   document.dispatchEvent(new CustomEvent(TOGGLE_DRAWER_EVENT));
 }
 
+/** Close the mobile drawer unconditionally — used after a spawn lands a new
+ *  session so the content pane is visible. Unlike TOGGLE, this is idempotent:
+ *  firing it when the drawer is already closed is a no-op. */
+export const CLOSE_DRAWER_EVENT = "maestro-close-drawer";
+
+export function requestCloseDrawer(): void {
+  document.dispatchEvent(new CustomEvent(CLOSE_DRAWER_EVENT));
+}
+
 /** Toggle the help overlay from a button click (the `?` key is handled by
  *  HelpOverlay's own command set). */
 export const TOGGLE_HELP_EVENT = "maestro-toggle-help";