refactor: modal-gate helper + data-driven colormap table

Two extensibility easy-wins, both behavior-preserving:

- UiModalOpen() centralizes the "an overlay owns input" check that was
  copy-pasted as `!showFileBrowser && !showAbout` across 6 input gates.
  New overlays now update one place.
- Colormaps become a COLORMAPS[] table (name + function) indexed by enum.
  GetColormapColor and the sidebar both read it, so adding a colormap is
  one enum value + one Cmap* fn + one row — and the sidebar name can no
  longer drift from the enum (deleted the parallel colormapNames[] array).

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

src/spectrogram_types.h

@@ -226,6 +226,13 @@ extern Font mainFont;
 // (defined in spectrogram.c; used by every load path).
 void ResetForNewSignal(void);
 
+// True when a modal overlay owns input; normal spectrogram/keyboard interaction
+// is gated off while this is the case. Add new overlays here in one place.
+static inline bool UiModalOpen(void)
+{
+    return app.showFileBrowser || app.showAbout;
+}
+
 // ============================================================================
 // Small math helpers (header-inline so every module can use them)
 // ============================================================================