feat(e2e): add test mode infrastructure with BTERMINAL_TEST env isolation

Rust: watcher.rs/fs_watcher.rs skip watchers in test mode,
is_test_mode Tauri command. Frontend: wake-scheduler disable,
App.svelte test mode detection. AppConfig centralization in
bterminal-core (OnceLock pattern for path overrides).

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

@@ -29,6 +29,11 @@ pub fn open_url(url: String) -> Result<(), String> {
     Ok(())
 }
 
+#[tauri::command]
+pub fn is_test_mode() -> bool {
+    std::env::var("BTERMINAL_TEST").map_or(false, |v| v == "1")
+}
+
 #[tauri::command]
 pub fn frontend_log(level: String, message: String, context: Option<serde_json::Value>) {
     match level.as_str() {