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.

flutter/lib/desktop/pages/desktop_setting_page.dart

@@ -10,6 +10,7 @@ import 'package:flutter_hbb/common/widgets/audio_input.dart';
 import 'package:flutter_hbb/common/widgets/setting_widgets.dart';
 import 'package:flutter_hbb/consts.dart';
 import 'package:flutter_hbb/desktop/pages/desktop_home_page.dart';
+import 'package:flutter_hbb/common/widgets/keyboard_shortcuts/page_body.dart';
 import 'package:flutter_hbb/desktop/pages/desktop_keyboard_shortcuts_page.dart';
 import 'package:flutter_hbb/desktop/pages/desktop_tab_page.dart';
 import 'package:flutter_hbb/desktop/widgets/remote_toolbar.dart';
@@ -459,6 +460,7 @@ class _GeneralState extends State<_General> {
               await ShortcutModel.setPassThrough(v);
               setLocalState(() {});
             },
+            trailing: const InfoTooltipIcon(tipKey: 'shortcut-passthrough-tip'),
           ),
           _ShortcutsConfigureRow(),
         ],
@@ -2532,6 +2534,8 @@ Widget _OptionCheckBox(
   bool isServer = true,
   bool Function()? optGetter,
   Future<void> Function(String, bool)? optSetter,
+  // Optional widget rendered between the label and the trailing space.
+  Widget? trailing,
 }) {
   getOpt() => optGetter != null
       ? optGetter()
@@ -2575,11 +2579,23 @@ Widget _OptionCheckBox(
             offstage: !ref.value || checkedIcon == null,
             child: checkedIcon?.marginOnly(right: 5),
           ),
-          Expanded(
+          // Without `trailing`, keep the original Expanded(Text) layout.
+          if (trailing == null)
+            Expanded(
+                child: Text(
+              translate(label),
+              style: TextStyle(color: disabledTextColor(context, enabled)),
+            ))
+          else ...[
+            Flexible(
               child: Text(
-            translate(label),
-            style: TextStyle(color: disabledTextColor(context, enabled)),
-          ))
+                translate(label),
+                style: TextStyle(color: disabledTextColor(context, enabled)),
+              ),
+            ),
+            trailing,
+            const Spacer(),
+          ],
         ],
       ),
     ).marginOnly(left: _kCheckBoxLeftMargin),