feat: keyboard and mouse shortcut overview window

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/shortcuts.hpp

@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: MIT
+#pragma once
+
+namespace udpscope {
+
+/// Everything a key can ask the application to do. The plot's own actions (zoom, pan, fit)
+/// are not here: they belong to the plotter's input controller, which handles its own keys.
+enum class AppAction {
+  PauseToggle,
+  TriggerArm,
+  TriggerPanel,
+  CursorsPane,
+  MeasuresPane,
+  MathPane,
+  LayoutEditor,
+  SidePanel,
+  Export,
+  Help,
+  Shortcuts,
+  Quit,
+};
+
+/// One application accelerator: what to press, what it does, and where the overview lists it.
+struct Accel {
+  unsigned keyval;         ///< GDK keyval
+  unsigned mods;           ///< GdkModifierType mask, 0 for a bare key
+  AppAction action;
+  const char* accel_text;  ///< what the overview prints
+  const char* description;
+  const char* section;
+};
+
+/// One documented plot interaction. These have no AppAction: the plotter's own controller
+/// implements them, and this table exists so the overview can list them.
+struct Gesture {
+  const char* accel_text;
+  const char* description;
+  const char* section;
+};
+
+/// The application accelerator table. Writes the row count to `count`.
+const Accel* app_accels(int* count);
+
+/// The plot keyboard and mouse table. Writes the row count to `count`.
+const Gesture* plot_gestures(int* count);
+
+/// The accelerator for `keyval` with `mods`, or nullptr. `mods` is masked to
+/// Ctrl/Shift/Alt before comparing, because Lock, Num-Lock and the pointer buttons
+/// arrive in the same mask and would otherwise never match a table row.
+const Accel* find_accel(unsigned keyval, unsigned mods);
+
+}  // namespace udpscope