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/ui.c

@@ -394,7 +394,6 @@ void DrawSidebar(void)
 
     // Colormap dropdown
     DrawTextScaled("Colormap:", x, y, 14, LIGHTGRAY); y += 20 * scale;
-    const char* colormapNames[] = { "Grays", "Inferno", "Viridis", "Plasma", "Hot", "Cool" };
     Rectangle cmapButton = { x, y, sidebarWidth - 10 * scale, 25 * scale };
     if (CheckCollisionPointRec(GetMousePosition(), cmapButton) && IsMouseButtonPressed(MOUSE_LEFT_BUTTON)) {
         app.colormap = (ColormapType)((app.colormap + 1) % COLORMAP_COUNT);
@@ -403,7 +402,7 @@ void DrawSidebar(void)
     }
     DrawRectangleRec(cmapButton, (Color){ 50, 50, 60, 255 });
     DrawRectangleLinesEx(cmapButton, 1, GRAY);
-    DrawTextScaled(colormapNames[app.colormap], cmapButton.x + 10 * scale, cmapButton.y + 6 * scale, 14, WHITE);
+    DrawTextScaled(ColormapName(app.colormap), cmapButton.x + 10 * scale, cmapButton.y + 6 * scale, 14, WHITE);
     DrawTexturePro(colormapTexture, (Rectangle){ 0, 0, 256, 1 },
                    (Rectangle){ cmapButton.x + cmapButton.width - 60 * scale, cmapButton.y + 5 * scale, 50 * scale, 15 * scale },
                    (Vector2){ 0, 0 }, 0.0f, WHITE);