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_file_cont.svelte

@@ -92,6 +92,7 @@
 
     async function handle_open_file() {
         if (log_lvl) console.log('*** handle_open_file() ***');
+        if (open_file_clicked) return; // Hard Guard: Already processing
 
         $events_slct.event_file_id = event_file_id;
         $events_slct.event_file_obj = event_file_obj;
@@ -107,7 +108,8 @@
 
             const exists = await native.check_hash_file_cache({
                 cache_root,
-                hash: event_file_obj.hash_sha256
+                hash: event_file_obj.hash_sha256,
+                verify_hash: true // Hardened: Trust No One!
             });
 
             if (!exists) {