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

README.md

@@ -24,10 +24,22 @@ rebuild switch   # zsh helper: sudo nixos-rebuild switch --flake ~/NixOS-Configs
 ## Adding a new host (e.g. a laptop)
 
 1. Create `hosts/<name>/` with `default.nix`, `hardware-configuration.nix`, and
-   (optionally) `disko.nix`; import the shared modules (`modules/core`,
-   `modules/graphical`, `modules/apps`).
+   (optionally) `disko.nix`. Shared modules (`modules/core`, `modules/graphical`,
+   `modules/apps`) are already pulled in by `commonModules` in `flake.nix` — the
+   host only adds its own hardware/disko config.
 2. Register it in `flake.nix` under `nixosConfigurations`.
-3. Declare host-specific displays in `hosts/<name>/default.nix`:
+3. Turn optional features on per machine via the shared `my.*` options:
+
+```nix
+# hosts/<name>/default.nix
+my = {
+  username = "anish";      # default; change for a different primary user
+  gaming.enable = false;   # Steam, Gamemode, MangoHud
+  ai.local.enable = false; # Ollama, ComfyUI, Open WebUI
+};
+```
+
+4. Declare host-specific displays in `hosts/<name>/default.nix`:
 
 ```nix
 home-manager.users.anish.volt.monitors = [
@@ -44,4 +56,19 @@ home-manager.users.anish.volt.monitors = [
 
 The `volt.monitors` default (any output @ preferred) keeps every machine working
 out of the box; hosts override only what they need. Hosts without an NVIDIA GPU
-simply don't import `modules/hardware/nvidia.nix` — all vendor env vars live there.
+simply don't import `modules/hardware/nvidia.nix` — all vendor env vars live there,
+while the vendor-neutral `hardware.graphics` stack lives in `modules/graphical`.
+
+## Feature toggles (`my.*`)
+
+Optional app modules are always imported (via `modules/apps/default.nix`) but only
+activate when a host sets the corresponding flag, so hosts never need to edit
+import lists:
+
+| Option | Enables |
+| --- | --- |
+| `my.username` | Primary user (users, home-manager, getty autologin) |
+| `my.gaming.enable` | Steam, Gamemode, MangoHud |
+| `my.ai.local.enable` | Ollama (CUDA), ComfyUI, Open WebUI |
+
+Hardware modules remain explicit imports per host (e.g. `modules/hardware/nvidia.nix`).