Rename fastfetch.nix -> settings/shell.nix, remove flatpaks

- Move shell/prompt/font config from apps/fastfetch.nix to settings/shell.nix
- Remove flatpaks.nix and nix-flatpak flake input (no flatpak packages in use)
- Update readme structure and flake inputs table

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

common.nix

@@ -13,8 +13,7 @@
     ./settings/locale.nix
     ./settings/audio.nix
     ./settings/users.nix
-    ./apps/fastfetch.nix
-    ./apps/flatpaks.nix
+    ./settings/shell.nix
     ./apps/zen.nix
 
     # Services #