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/styles.scss

@@ -360,6 +360,22 @@ canvas {
     &.M2 {
       background-color: green;
     }
+
+    &.color1 {
+      background-color: #FF5252;
+    }
+
+    &.color2 {
+      background-color: #448AFF;
+    }
+
+    &.color3 {
+      background-color: #4CAF50;
+    }
+
+    &.color4 {
+      background-color: #FFEB3B;
+    }
   }
 }