Add Four-Color algorithm page

Introduce an interactive Four Color Theorem demo: add FourColorComponent (template, styles, standalone component) with canvas-based grid, region generation, adjacency detection and a backtracking solver; include models for nodes/regions and grid constants. Wire up route and RouterConstants/UrlConstants, add menu entry in AlgorithmsService, and add i18n entries for English and German. Also add global color styles in styles.scss and a few minor i18n text adjustments.

src/app/constants/RouterConstants.ts

@@ -55,6 +55,11 @@
     LINK: '/algorithms/cloth',
   };
 
+  static readonly FOUR_COLOR = {
+    PATH: 'algorithms/four_color',
+    LINK: '/algorithms/four_color',
+  };
+
   static readonly IMPRINT = {
     PATH: 'imprint',
     LINK: '/imprint',