refactor: table-driven keymap dispatch + auto-derived help list

Replace the global key handling scattered through the main loop with a single
KEYMAP table (key, gate, action, help text) and a DispatchKeymap() pass. The
About/Help overlay now renders its key list straight from the table, so the
on-screen shortcuts can never drift from the actual bindings.

- Order-sensitive keys (Space, Esc) stay inline where their frame ordering
  matters; they carry action==NULL and appear in the table for the help list.
- Adds the F11 fullscreen binding the sidebar button already advertised but
  that was never actually wired up.
- Fix About title em-dash rendering as '?' (font is ASCII-only); use a hyphen.

Launch render verified pixel-identical (AE=0); keys verified via injected input.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/spectrogram_types.h

@@ -233,6 +233,32 @@ static inline bool UiModalOpen(void)
     return app.showFileBrowser || app.showAbout;
 }
 
+// ============================================================================
+// Keymap — single source of truth for global key bindings.
+// The dispatcher (DispatchKeymap in spectrogram.c) runs every entry whose
+// `action` is non-NULL and whose gate passes; the About/Help overlay renders
+// the whole table so the on-screen key list can never drift from the bindings.
+// Order-sensitive keys (Space, Esc) carry action==NULL and are handled inline
+// where their frame ordering matters; they appear here for documentation only.
+// ============================================================================
+typedef void (*KeyActionFn)(void);
+
+#define KEYGATE_NONE   0u
+#define KEYGATE_MODAL  1u   // skip while a modal overlay is open (!UiModalOpen())
+#define KEYGATE_LOADED 2u   // require a loaded signal
+#define KEYGATE_STFT   4u   // require a computed STFT
+
+typedef struct {
+    int key;             // raylib key code
+    unsigned gate;       // KEYGATE_* bitmask
+    KeyActionFn action;  // NULL = handled inline / documentation-only
+    const char* label;   // shown in the help overlay, e.g. "O", "Home"
+    const char* help;    // description for the help overlay
+} KeyBinding;
+
+// Returns the keymap table and its entry count (defined in spectrogram.c).
+const KeyBinding* GetKeymap(int* count);
+
 // ============================================================================
 // Small math helpers (header-inline so every module can use them)
 // ============================================================================