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.

documentation/SVELTE_DEXIE_GUIDE.md

@@ -92,6 +92,35 @@ export function createLiveQueryStore<T>(query: () => T | Promise<T>) {
 
 The `createLiveQueryStore` function creates a readable store that automatically updates whenever the data in the `friends` table changes. The `$friends` variable in the component will always contain the latest data from the database.
 
+## Svelte 5 Binding Pitfalls
+
+### 1. `props_invalid_value` (The "Expression Binding" Error)
+Svelte 5's `bind:` directive is more restrictive than previous versions. You can only bind to a simple **Identifier** or **MemberExpression**.
+
+**❌ Invalid Pattern (Causes Compile Error):**
+Attempting to normalize a value *inside* the binding will fail.
+```svelte
+<!-- Error: Can only bind to an Identifier or MemberExpression -->
+<Launcher_menu bind:slct__event_session_id={$events_slct.event_session_id || null} />
+```
+
+**✅ Correct Pattern:**
+Ensure the source value is already normalized before binding, or use a reactive effect to handle the fallback.
+```typescript
+// Normalize in an effect or derivation
+$effect(() => {
+    if ($events_slct.event_session_id === undefined) {
+        $events_slct.event_session_id = null;
+    }
+});
+```
+```svelte
+<!-- Bind directly to the normalized property -->
+<Launcher_menu bind:slct__event_session_id={$events_slct.event_session_id} />
+```
+
+---
+
 ## Safe Data Processing for IndexedDB Sorting
 
 When preparing data for IndexedDB, especially when creating composite sort keys, it is critical to handle `null` or `undefined` values safely to prevent runtime crashes that can interrupt the data synchronization process.