[Launcher] Fix $bindable warning on slct_event_location_id; audit TODO items

- menu_location_list.svelte: mark slct_event_location_id as $bindable(null) to
  resolve Svelte 5 compiler warning (bind:value used on non-bindable prop)
- TODO__Agents.md: audit and close resolved launcher items:
  - Location select auto-load bug: fixed via $derived.by() liveQuery pattern
  - Session Search button visibility: was never a real bug, hardcoded false
  - Dark mode select fix: already applied via app.css color-scheme rules

src/routes/events/[event_id]/(launcher)/menu_location_list.svelte

@@ -2,7 +2,7 @@
     interface Props {
         loading__session_li_status?: null | boolean | string;
         lq__event_location_obj_li: any;
-        slct_event_location_id: string | null;
+        slct_event_location_id?: string | null;
 
         trigger_reload__event_session_obj_li?: boolean;
         trigger_reload__event_location_obj_li?: boolean;
@@ -13,7 +13,7 @@
     let {
         loading__session_li_status = $bindable(null),
         lq__event_location_obj_li,
-        slct_event_location_id = null,
+        slct_event_location_id = $bindable(null),
 
         trigger_reload__event_session_obj_li = $bindable(false),
         trigger_reload__event_location_obj_li = $bindable(false),