fix(installer): unblock disko, bootloader, HM activation, hyprland res

- Pass --yes-wipe-all-disks to disko so the silent gum-spin path no
  longer hangs forever waiting on a hidden "yes" confirmation prompt
  (added in disko 1.13's destroy,format,mount mode).
- Stop threading an externally-built pkgs into the user flake's
  nixosSystem; configure nixpkgs through the module system instead so
  core/system/default.nix's nixpkgs.config.allowUnfree stops conflicting
  with the assertion "system configures nixpkgs with an externally
  created instance".
- Enable boot.loader.systemd-boot in the generated system.nix so the
  installed system has an actual bootloader (disko already lays out a
  1 GiB ESP at /boot).
- Bump nix.settings.download-buffer-size to 512 MiB to silence the
  "download buffer is full" warning on large NAR fetches.
- Activate home-manager via `runuser -l` instead of `runuser -u … --
  env HOME=…`. The latter only switches uid and leaves \$USER=root, so
  HM's activation script saw root, warned, and wrote dotfiles into
  /root/ — meaning the user's first login had no Hyprland config.
- Revert default Hyprland monitor line back to highres (live ISO and
  user default) — preferred falls back to EDID's 1024x768 in QEMU and
  on several laptop panels, which is the bug highres was put there to
  defeat.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/system/nix.nix

@@ -11,6 +11,14 @@
       
       # Optimize storage by hard-linking identical files
       auto-optimise-store = lib.mkDefault true;
+
+      # Bump the substituter download buffer from the 64 MiB default to
+      # 512 MiB. Large NARs (kernels, electron apps, the full NixOS
+      # closure on first install) overrun 64 MiB and Nix prints
+      # "download buffer is full" until the consumer catches up — the
+      # warning is harmless but slows substitution and looks like an
+      # error. 512 MiB comfortably covers everything in our closure.
+      download-buffer-size = 524288000;
     };
 
     # Populates NIX_PATH with the nixpkgs input from the flake.