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/ui_session_interface.rs

@@ -23,7 +23,7 @@ use hbb_common::{
         sync::mpsc,
         time::{Duration as TokioDuration, Instant},
     },
-    whoami, Stream,
+    whoami, SessionID, Stream,
 };
 use rdev::{Event, EventType::*, KeyCode};
 #[cfg(all(feature = "vram", feature = "flutter"))]
@@ -913,6 +913,7 @@ impl<T: InvokeUiSession> Session<T> {
     #[cfg(any(target_os = "ios"))]
     pub fn handle_flutter_raw_key_event(
         &self,
+        _session_id: SessionID,
         _keyboard_mode: &str,
         _name: &str,
         _platform_code: i32,
@@ -925,6 +926,7 @@ impl<T: InvokeUiSession> Session<T> {
     #[cfg(not(any(target_os = "ios")))]
     pub fn handle_flutter_raw_key_event(
         &self,
+        session_id: SessionID,
         keyboard_mode: &str,
         name: &str,
         platform_code: i32,
@@ -936,6 +938,7 @@ impl<T: InvokeUiSession> Session<T> {
             self._handle_key_flutter_simulation(keyboard_mode, platform_code, down_or_up);
         } else {
             self._handle_raw_key_non_flutter_simulation(
+                session_id,
                 keyboard_mode,
                 platform_code,
                 position_code,
@@ -948,6 +951,7 @@ impl<T: InvokeUiSession> Session<T> {
     #[cfg(not(any(target_os = "ios")))]
     fn _handle_raw_key_non_flutter_simulation(
         &self,
+        session_id: SessionID,
         keyboard_mode: &str,
         platform_code: i32,
         position_code: i32,
@@ -981,11 +985,18 @@ impl<T: InvokeUiSession> Session<T> {
             #[cfg(any(target_os = "windows", target_os = "macos"))]
             extra_data: 0,
         };
-        keyboard::client::process_event_with_session(keyboard_mode, &event, Some(lock_modes), self);
+        keyboard::client::process_event_with_session(
+            keyboard_mode,
+            &event,
+            Some(lock_modes),
+            self,
+            session_id,
+        );
     }
 
     pub fn handle_flutter_key_event(
         &self,
+        session_id: SessionID,
         keyboard_mode: &str,
         character: &str,
         usb_hid: i32,
@@ -996,6 +1007,7 @@ impl<T: InvokeUiSession> Session<T> {
             self._handle_key_flutter_simulation(keyboard_mode, usb_hid, down_or_up);
         } else {
             self._handle_key_non_flutter_simulation(
+                session_id,
                 keyboard_mode,
                 character,
                 usb_hid,
@@ -1031,6 +1043,7 @@ impl<T: InvokeUiSession> Session<T> {
 
     fn _handle_key_non_flutter_simulation(
         &self,
+        session_id: SessionID,
         keyboard_mode: &str,
         character: &str,
         usb_hid: i32,
@@ -1092,7 +1105,13 @@ impl<T: InvokeUiSession> Session<T> {
             #[cfg(any(target_os = "windows", target_os = "macos"))]
             extra_data: 0,
         };
-        keyboard::client::process_event_with_session(keyboard_mode, &event, Some(lock_modes), self);
+        keyboard::client::process_event_with_session(
+            keyboard_mode,
+            &event,
+            Some(lock_modes),
+            self,
+            session_id,
+        );
     }
 
     // flutter only TODO new input