Add a Nix flake (NixOS, GNOME and Hyprland)

Builds the desktop app and the CLI from source with `nix build`, and offers
packages, an overlay, `nix run`, a dev shell and a formatting check.

- Cargo.lock drives the dependencies (git sources via allowBuiltinFetchGit).
- Installs the desktop file, hicolor icons, MIME types and AppStream metadata
  from packaging/linux/, validated at build time.
- Wraps the binaries so the dlopen'd libraries (libwayland-client,
  libxkbcommon(-x11), X11, Vulkan loader, libEGL, libdbus for the portal file
  chooser) are found on NixOS: native Wayland on GNOME and Hyprland, X11 as
  fallback.
- Fonts from storytold/craft-fonts are pinned to the release workflow's revision
  where the app embeds them.
- doCheck is off (the suite needs corpora and a GPU); `nix build` was verified.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1791594108,
+        "narHash": "sha256-XK4KceonkwD8LuMAt/vNrz982mXJM5jeHAxMwsUXXAQ=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "6510408d147d8c5e6d76043e899e9251756f6a75",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}