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/UrlConstants.ts

@@ -23,5 +23,6 @@
   static readonly XPBD_WIKI = 'https://www.emergentmind.com/topics/extended-position-based-dynamics-xpbd'
   static readonly GPU_COMPUTING_WIKI = 'https://en.wikipedia.org/wiki/General-purpose_computing_on_graphics_processing_units'
   static readonly DATA_STRUCTURE_WIKI = 'https://de.wikipedia.org/wiki/Datenstruktur'
+  static readonly FOUR_COLOR_THEOREM = 'https://de.wikipedia.org/wiki/Vier-Farben-Satz'
 
 }