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/render.h

@@ -11,6 +11,7 @@ float MeasureTextScaled(const char* text, float baseSize);
 
 // --- Colormaps ---
 void GenerateColormapTexture(void);
+const char* ColormapName(ColormapType type);
 
 // --- Spectrogram texture ---
 // GenerateSpectrogramTexture recomputes the synchrosqueezed reassignment (use