fix(launcher): resolve hydration errors and missing file counts

- Initialized event_session_id to null in store template to prevent Svelte 5 binding errors.\n- Fixed invalid bind expressions in Launcher layout.\n- Corrected view: alt propagation in session list background refresh to ensure file counts are retrieved.\n- Hardened duplicate launch protection with verify_hash enabled by default.\n- Updated SVELTE_DEXIE_GUIDE.md with binding pitfall documentation.

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

@@ -1,13 +1,13 @@
 <script lang="ts">
     interface Props {
-        slct__event_session_id?: string;
+        slct__event_session_id?: string | null;
         lq__event_session_obj: any;
         type_code?: string;
         log_lvl?: number;
     }
 
     let {
-        slct__event_session_id = $bindable(''),
+        slct__event_session_id = $bindable(null),
         lq__event_session_obj,
         type_code = $bindable(''),
         log_lvl = $bindable(1)