chore: get nix flake check evaluation green

Three blockers off the path so the skeleton evaluates cleanly:

1. flake.lock generated (no inputs were pinned before).
2. hardware-configuration.nix shipped as a placeholder — unbootable
   /dev/null root + grub on nodev — so `system.build.toplevel` resolves
   without complaining about missing fileSystems / boot.loader. The
   file carries a big "overwrite with nixos-generate-config" warning;
   the consumer regenerates it before any real switch.
3. Primary user + group declared in configuration.nix (was tripping
   the assertion about implicit nogroup defaults and isNormalUser
   missing).

Also: `home.homeDirectory` now uses `lib.mkForce` to override
home-manager's nixos-module default of /var/empty, and the deprecated
`programs.git.user{Name,Email}` options migrated to the new
`programs.git.settings.user.{name,email}` shape.

`nix flake check --no-build` now finishes with `all checks passed!`
and no warnings, giving every subsequent change a cheap signal.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,65 @@
+{
+  "nodes": {
+    "home-manager": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1779678629,
+        "narHash": "sha256-gHcIFg0mm+KFsg7iZQt67kni3+qR5U3PhEC9P7vKlZ4=",
+        "owner": "nix-community",
+        "repo": "home-manager",
+        "rev": "612bbe3b405ad5f71d7bf9edecc04b678a061652",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "home-manager",
+        "type": "github"
+      }
+    },
+    "lnbits-src": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1779693880,
+        "narHash": "sha256-Gp+Dwj+zxMS7HaNksazfygMIFrAwW1nyWlNvKnaUta8=",
+        "owner": "lnbits",
+        "repo": "lnbits",
+        "rev": "88672501d8dbb3a77e2ebb5dfc7878dc43adadbd",
+        "type": "github"
+      },
+      "original": {
+        "owner": "lnbits",
+        "repo": "lnbits",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1779508470,
+        "narHash": "sha256-Ap9KJX+5xHIn3bPIpfNgT6MEXdAECECwo4/rmlQD74M=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "29916453413845e54a65b8a1cf996842300cd299",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "home-manager": "home-manager",
+        "lnbits-src": "lnbits-src",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}