refactor: add 6 color constants, replace 61 inline literals, remove const_cast

- Add kBrightGold, kPaleRed, kBrightRed, kLightBlue, kManaBlue, kCyan to ui_colors.hpp
- Replace 61 inline ImVec4 color literals across game_screen, inventory_screen,
  talent_screen, and world_map with named constants
- Remove const_cast in character_renderer render loop by using non-const iteration

include/ui/ui_colors.hpp

@@ -16,6 +16,12 @@ namespace colors {
     constexpr ImVec4 kLightGray   = {0.7f, 0.7f, 0.7f, 1.0f};
     constexpr ImVec4 kWhite       = {1.0f, 1.0f, 1.0f, 1.0f};
     constexpr ImVec4 kTooltipGold = {1.0f, 0.82f, 0.0f, 1.0f};
+    constexpr ImVec4 kBrightGold  = {1.0f, 0.85f, 0.0f, 1.0f};
+    constexpr ImVec4 kPaleRed     = {1.0f, 0.5f, 0.5f, 1.0f};
+    constexpr ImVec4 kBrightRed   = {1.0f, 0.2f, 0.2f, 1.0f};
+    constexpr ImVec4 kLightBlue   = {0.4f, 0.6f, 1.0f, 1.0f};
+    constexpr ImVec4 kManaBlue    = {0.2f, 0.2f, 0.9f, 1.0f};
+    constexpr ImVec4 kCyan        = {0.0f, 0.8f, 1.0f, 1.0f};
 
     // Coin colors
     constexpr ImVec4 kGold   = {1.00f, 0.82f, 0.00f, 1.0f};