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>

flake.lock

@@ -109,21 +109,6 @@
         "type": "github"
       }
     },
-    "nix-flatpak": {
-      "locked": {
-        "lastModified": 1768656715,
-        "narHash": "sha256-Sbh037scxKFm7xL0ahgSCw+X2/5ZKeOwI2clqrYr9j4=",
-        "owner": "gmodena",
-        "repo": "nix-flatpak",
-        "rev": "123fe29340a5b8671367055b75a6e7c320d6f89a",
-        "type": "github"
-      },
-      "original": {
-        "owner": "gmodena",
-        "repo": "nix-flatpak",
-        "type": "github"
-      }
-    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1775231746,
@@ -175,7 +160,6 @@
       "inputs": {
         "home-manager": "home-manager",
         "nix-cachyos-kernel": "nix-cachyos-kernel",
-        "nix-flatpak": "nix-flatpak",
         "nixpkgs": "nixpkgs_2",
         "zen-browser": "zen-browser"
       }