feat: add FTS5 full-text search with Spotlight-style overlay

Upgrade rusqlite to bundled-full for FTS5. SearchDb with 3 virtual tables
(messages, tasks, btmsg). SearchOverlay.svelte: Ctrl+Shift+F, 300ms
debounce, grouped results with highlight snippets.

v2/src-tauri/src/commands/search.rs

@@ -0,0 +1,35 @@
+use crate::AppState;
+use crate::search::SearchResult;
+use tauri::State;
+
+#[tauri::command]
+pub fn search_init(state: State<'_, AppState>) -> Result<(), String> {
+    // SearchDb is already initialized during app setup; this is a no-op
+    // but allows the frontend to confirm readiness.
+    let _db = &state.search_db;
+    Ok(())
+}
+
+#[tauri::command]
+pub fn search_query(
+    state: State<'_, AppState>,
+    query: String,
+    limit: Option<i32>,
+) -> Result<Vec<SearchResult>, String> {
+    state.search_db.search_all(&query, limit.unwrap_or(20))
+}
+
+#[tauri::command]
+pub fn search_rebuild(state: State<'_, AppState>) -> Result<(), String> {
+    state.search_db.rebuild_index()
+}
+
+#[tauri::command]
+pub fn search_index_message(
+    state: State<'_, AppState>,
+    session_id: String,
+    role: String,
+    content: String,
+) -> Result<(), String> {
+    state.search_db.index_message(&session_id, &role, &content)
+}