refactor: make hosts declarative and consolidate shared config

- add my.username / my.gaming.enable / my.ai.local.enable options and
  gate optional app modules on them instead of per-host import lists
- move vendor-neutral hardware.graphics into modules/graphical, leaving
  only NVIDIA-specific drivers and env vars in modules/hardware/nvidia
- derive the primary user everywhere from my.username (users, getty,
  home-manager, tmpfiles)
- de-duplicate Hyprland autostart down to a single system loginShellInit
  using the start-hyprland wrapper
- consolidate all nixpkgs overlays into modules/core/nix.nix and use
  final.stdenv.hostPlatform.system
- make the zsh rebuild helper target $(hostname) instead of hardcoded hades
- add nixpkgs follows for nixos-hardware, pi and comfy-ui (single nixpkgs)
- add .gitignore and a nixfmt flake formatter; format all nix files
- remove empty browser.nix/display-configuration.nix and rename
  local_ai.nix -> local-ai.nix
- document the my.* toggles in the README

flake.lock

@@ -51,7 +51,9 @@
     "comfy-ui": {
       "inputs": {
         "flake-parts": "flake-parts",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": [
+          "nixpkgs"
+        ]
       },
       "locked": {
         "lastModified": 1787851822,
@@ -186,7 +188,9 @@
     },
     "nixos-hardware": {
       "inputs": {
-        "nixpkgs": "nixpkgs_2"
+        "nixpkgs": [
+          "nixpkgs"
+        ]
       },
       "locked": {
         "lastModified": 1788044418,
@@ -205,16 +209,16 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1766902085,
-        "narHash": "sha256-coBu0ONtFzlwwVBzmjacUQwj3G+lybcZ1oeNSQkgC0M=",
+        "lastModified": 1787962033,
+        "narHash": "sha256-u6z9VTZA4Kf3RkHQo9sQI7NI4Ei/uiU9vrMqOiwWP1Y=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "c0b0e0fddf73fd517c3471e546c0df87a42d53f4",
+        "rev": "c5c4a43b0e8056328ec4529f735cabdb8f1942bb",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
-        "ref": "nixos-unstable",
+        "ref": "nixos-26.05",
         "repo": "nixpkgs",
         "type": "github"
       }
@@ -250,56 +254,13 @@
         "type": "github"
       }
     },
-    "nixpkgs_2": {
-      "locked": {
-        "lastModified": 1767892417,
-        "narHash": "sha256-8bW3q88CEg2u4hSP66Vf4lpbLonHz7hqDNBMcCY7E9U=",
-        "rev": "3497aa5c9457a9d88d71fa93a4a8368816fbeeba",
-        "type": "tarball",
-        "url": "https://releases.nixos.org/nixos/unstable/nixos-26.05pre924538.3497aa5c9457/nixexprs.tar.xz"
-      },
-      "original": {
-        "type": "tarball",
-        "url": "https://channels.nixos.org/nixos-unstable/nixexprs.tar.xz"
-      }
-    },
-    "nixpkgs_3": {
-      "locked": {
-        "lastModified": 1787962033,
-        "narHash": "sha256-u6z9VTZA4Kf3RkHQo9sQI7NI4Ei/uiU9vrMqOiwWP1Y=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "c5c4a43b0e8056328ec4529f735cabdb8f1942bb",
-        "type": "github"
-      },
-      "original": {
-        "owner": "NixOS",
-        "ref": "nixos-26.05",
-        "repo": "nixpkgs",
-        "type": "github"
-      }
-    },
-    "nixpkgs_4": {
-      "locked": {
-        "lastModified": 1773646010,
-        "narHash": "sha256-iYrs97hS7p5u4lQzuNWzuALGIOdkPXvjz7bviiBjUu8=",
-        "owner": "nixos",
-        "repo": "nixpkgs",
-        "rev": "5b2c2d84341b2afb5647081c1386a80d7a8d8605",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nixos",
-        "ref": "nixos-unstable",
-        "repo": "nixpkgs",
-        "type": "github"
-      }
-    },
     "pi": {
       "inputs": {
         "bun2nix": "bun2nix",
         "jail-nix": "jail-nix",
-        "nixpkgs": "nixpkgs_4",
+        "nixpkgs": [
+          "nixpkgs"
+        ],
         "systems": "systems"
       },
       "locked": {
@@ -324,7 +285,7 @@
         "herdr": "herdr",
         "home-manager": "home-manager",
         "nixos-hardware": "nixos-hardware",
-        "nixpkgs": "nixpkgs_3",
+        "nixpkgs": "nixpkgs",
         "nixpkgs-unstable": "nixpkgs-unstable",
         "pi": "pi"
       }