feat(shortcuts): user-configurable keyboard shortcuts for session actions

Adds a keyboard shortcut feature (Rust matcher + Dart UI + cross-language
  parity tests) that lets users bind combinations like Ctrl+Alt+Shift+P to
  session actions. Bindings are stored in LocalConfig under
  `keyboard-shortcuts`; the matcher gates dispatch on `enabled` and
  `pass_through` flags so flipping the master switch off is a hard stop.

  Wire-up summary:
  - src/keyboard/shortcuts.rs: matcher, default bindings, parity test against
    flutter/test/fixtures/default_keyboard_shortcuts.json
  - src/keyboard.rs: shortcut intercept in process_event{,_with_session},
    feature-gated to `flutter`; runs before key swapping so users bind to
    physical keys
  - src/flutter_ffi.rs: main_reload_keyboard_shortcuts +
    main_get_default_keyboard_shortcuts; reload_from_config seeded in main_init
  - flutter/lib/common/widgets/keyboard_shortcuts/: shared config page body,
    recording dialog, shortcut display formatter, action group registry
  - flutter/lib/desktop/pages/desktop_keyboard_shortcuts_page.dart and
    flutter/lib/mobile/pages/mobile_keyboard_shortcuts_page.dart: platform
    shells around the shared body
  - flutter/lib/models/shortcut_model.dart: per-session ShortcutModel +
    registerSessionShortcutActions for actions with no toolbar TToggleMenu /
    TRadioMenu (fullscreen, switch display/tab, close tab, voice call, etc.)
  - flutter/lib/common/widgets/toolbar.dart: optional `actionId` field on
    TToggleMenu / TRadioMenu, plus per-helper auto-register pass that wires
    tagged entries' existing onChanged into the ShortcutModel
  - flutter/test/keyboard_shortcuts_test.dart + fixtures: cross-language
    parity (default bindings, supported key vocabulary)

  Design principles applied during review:

  1. Additions are fine; modifications to original logic must be deliberate.
     Tagging an existing TToggleMenu entry with `actionId:` is an addition.
     Rewriting its onChanged to satisfy a new contract is a modification —
     and was reverted for every case where the original click behavior was
     working. Four closures were touched and then reverted (mobile View
     Mode, Privacy mode multi-impl, Relative mouse mode, Reverse mouse
     wheel); their shortcuts are wired via standalone closures in
     shortcut_model.dart instead.

  2. Toolbar auto-register is reserved for entries whose onChanged is
     inherently self-flipping — typically `sessionToggleOption(name)` where
     the named option is flipped in place and the input bool is unused. The
     register pass passes `!menu.value` from registration time, which is
     harmless under self-flipping but wrong for closures that consume the
     input bool directly. Tagging a non-self-flipping entry forces a closure
     rewrite; choose non-toolbar registration in that case.

  3. When shortcuts are disabled, toolbar behavior must be bit-for-bit
     unchanged. The matcher's `enabled`-gate already guarantees no
     dispatch; the auto-register pass is left unconditional (its only effect
     is HashMap operations on a separate ShortcutModel) so mid-session
     enable works without a reconnect. The trade-off is intentional and
     documented at the top of toolbarControls.

  4. Comments stay terse. Rationale lives in one place — the doc comment of
     the helper or registration site, not duplicated at every call site.

  5. Where an existing helper needs a new optional behavior (e.g.
     `_OptionCheckBox` gaining a tooltip slot), the new branch must reduce
     to byte-identical output for existing callers (`trailing == null`
     case → original `Expanded(Text)` layout). Verified.

  6. Action IDs and labels stay consistent. Renamed `reset_cursor` →
     `reset_canvas` so the action ID matches its user-facing label
     ("Reset canvas") and capability flag.

  Out-of-scope but included:
  - AGENTS.md: documents flutter_rust_bridge no-codegen workflow and the
    Web target's hand-written TS client, since both are load-bearing for
    any new FFI work.
  - remote_toolbar.dart: i18n fix for the per-monitor tooltip ("All
    monitors" / "Monitor #N"), unrelated to shortcuts but kept here.

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())
 }