feat(fourier): add epicycles algorithm demo

Draw a shape or pick a preset, then rebuild it with a direct DFT.
Each frequency becomes one circle. Circles are sorted by radius, and
the circles slider keeps the biggest N. Also tick the ROADMAP item and
add the FOURIER SEO key.

src/app/constants/UrlConstants.ts

@@ -24,6 +24,7 @@
   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'
+  static readonly FOURIER_SERIES_WIKI = 'https://de.wikipedia.org/wiki/Fourierreihe'
   static readonly GITEA = 'https://git.andreas-dahm.eu'
 
 }