feat: keyboard shortcuts in remote sessions

Add an opt-in keyboard-shortcut system that triggers session
actions (Send Ctrl+Alt+Del, Toggle Fullscreen, Switch Display,
Screenshot, Switch Tab, etc.) via three-modifier combinations
during a remote session.

Architecture
- Native: src/keyboard/shortcuts.rs intercepts at the encoder
  layer (process_event and process_event_with_session), so the
  feature is input-source-independent. Bindings persist as a
  single JSON blob in LocalConfig.
- Web: matching + keydown intercept live in the separate hand-
  written TS client at flutter/web/js/ (gitignored, not in this
  repo). flutter/lib/web/bridge.dart::mainInit registers
  window.onShortcutTriggered so the JS matcher can dispatch
  back into the active session's ShortcutModel; the bridge's
  mainReloadKeyboardShortcuts forwards to a JS reloadShortcuts
  on settings writes.
- Three-modifier prefix (Ctrl+Alt+Shift; Cmd+Option+Shift on
  macOS/iOS) sidesteps the need for a pass-through toggle.
- Flutter native path threads the explicit per-call SessionID
  for tab-precise routing; rdev path uses globally-current
  session.

UI
- Settings -> General -> Keyboard Shortcuts opens a dedicated
  configuration page; desktop and mobile share a body widget.
- Recording dialog with live capture, prefix validation, and a
  conflict-replace flow.
- Toolbar menu items display the bound shortcut inline.
- Default bindings (adapted from AnyDesk):
    +Del    Send Ctrl+Alt+Del
    +Enter  Toggle Fullscreen
    +Left/Right  Switch Display Prev/Next
    +P      Screenshot
    +1..9   Switch Session Tab

Other
- AGENTS.md: documented (a) flutter_rust_bridge_codegen needs
  a pinned version + Dart bridge wrappers should be hand-
  written, and (b) the Web-target split where flutter/web/js/
  is the runtime owner on Web rather than wasm-compiled Rust.
- 38 new i18n strings in src/lang/en.rs with Chinese
  translations in src/lang/cn.rs.

Refs discussion #1933.

src/flutter_ffi.rs

@@ -575,6 +575,7 @@ pub fn session_handle_flutter_key_event(
     if let Some(session) = sessions::get_session_by_session_id(&session_id) {
         let keyboard_mode = session.get_keyboard_mode();
         session.handle_flutter_key_event(
+            session_id,
             &keyboard_mode,
             &character,
             usb_hid,
@@ -595,6 +596,7 @@ pub fn session_handle_flutter_raw_key_event(
     if let Some(session) = sessions::get_session_by_session_id(&session_id) {
         let keyboard_mode = session.get_keyboard_mode();
         session.handle_flutter_raw_key_event(
+            session_id,
             &keyboard_mode,
             &name,
             platform_code,
@@ -1728,6 +1730,7 @@ pub fn cm_get_clients_length() -> usize {
 
 pub fn main_init(app_dir: String, custom_client_config: String) {
     initialize(&app_dir, &custom_client_config);
+    crate::keyboard::shortcuts::reload_from_config();
 }
 
 pub fn main_device_id(id: String) {
@@ -2247,6 +2250,17 @@ pub fn main_init_input_source() -> SyncReturn<()> {
     SyncReturn(())
 }
 
+pub fn main_reload_keyboard_shortcuts() -> SyncReturn<()> {
+    crate::keyboard::shortcuts::reload_from_config();
+    SyncReturn(())
+}
+
+pub fn main_get_default_keyboard_shortcuts() -> SyncReturn<String> {
+    let bindings = crate::keyboard::shortcuts::default_bindings();
+    let json = serde_json::to_string(&bindings).unwrap_or_default();
+    SyncReturn(json)
+}
+
 pub fn main_is_installed_lower_version() -> SyncReturn<bool> {
     SyncReturn(is_installed_lower_version())
 }