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/keyboard.rs

@@ -10,6 +10,7 @@ use crate::{client::get_key_state, common::GrabState};
 #[cfg(not(any(target_os = "android", target_os = "ios")))]
 use hbb_common::log;
 use hbb_common::message_proto::*;
+use hbb_common::SessionID;
 #[cfg(any(target_os = "windows", target_os = "macos"))]
 use rdev::KeyCode;
 use rdev::{Event, EventType, Key};
@@ -79,6 +80,8 @@ lazy_static::lazy_static! {
     };
 }
 
+pub mod shortcuts;
+
 pub mod client {
     use super::*;
 
@@ -319,6 +322,32 @@ pub mod client {
     }
 
     pub fn process_event(keyboard_mode: &str, event: &Event, lock_modes: Option<i32>) {
+        // Shortcut intercept — must come before any wire encoding.
+        // Only fires on KeyPress (event_to_key_name in shortcuts.rs returns None
+        // for KeyRelease and other non-press events), so flushed releases from
+        // release_remote_keys pass straight through to the encode/forward path.
+        //
+        // NOTE: Shortcut matching intentionally happens BEFORE any key swapping
+        // (swap_modifier_key) so that shortcuts bind to the physical keys pressed,
+        // not the swapped keys. This makes shortcut setup intuitive: users bind
+        // shortcuts to the actual keys they press, regardless of swap settings.
+        // Key swapping only affects what gets sent to the remote.
+        //
+        // Gated on `feature = "flutter"` because the dispatch target
+        // (`flutter::push_session_event`) is Flutter-only. Sciter builds never
+        // call `reload_from_config`, so the cache stays disabled and the
+        // matcher would no-op anyway — but we still skip the call entirely so
+        // a hand-edited config can't silently swallow keys on a UI that has
+        // no way to surface the action.
+        //
+        // `None` for session_id makes the helper resolve through
+        // `flutter::get_cur_session_id()` — the rdev grab loop is process-wide
+        // and has no per-event session context to thread.
+        #[cfg(feature = "flutter")]
+        if crate::keyboard::shortcuts::try_dispatch(None, event) {
+            return;
+        }
+
         let keyboard_mode = get_keyboard_mode_enum(keyboard_mode);
         if is_long_press(&event) {
             return;
@@ -334,7 +363,20 @@ pub mod client {
         event: &Event,
         lock_modes: Option<i32>,
         session: &Session<T>,
+        session_id: SessionID,
     ) {
+        // Shortcut intercept — see the long comment in `process_event` above
+        // for the KeyPress-only / feature-gate rationale. The only difference
+        // here is that the Flutter FFI path threads an explicit SessionID
+        // through, so dispatch targets the exact tab the keystroke originated
+        // from — no dependency on the global focus tracker.
+        #[cfg(feature = "flutter")]
+        if crate::keyboard::shortcuts::try_dispatch(Some(&session_id), event) {
+            return;
+        }
+        #[cfg(not(feature = "flutter"))]
+        let _ = session_id;
+
         let keyboard_mode = get_keyboard_mode_enum(keyboard_mode);
         if is_long_press(&event) {
             return;