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/bterminal-core/src/lib.rs

@@ -1,3 +1,4 @@
+pub mod config;
 pub mod event;
 pub mod pty;
 pub mod sidecar;