feat(packaging): add a Nix flake and NixOS module

Builds the four binaries with rustPlatform.buildRustPackage from
Cargo.lock, installs the desktop entries, icon, user service and
translations as the other packages do, and wraps the binaries with
exiftool and ffmpeg on PATH. The NixOS module installs the package,
pulls proton-drive.service into the graphical session with the
fusermount3 wrapper on its PATH, and enables FUSE and GNOME Keyring.

A Nix workflow builds the flake when its recipe, the lock files or the
code change.

Refs #37

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EuyxByBpK3fSWtyPkugeHz

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1791456040,
+        "narHash": "sha256-H83yz/do+hjOO0uujHyH8RgtIQP2v7Mw+cLwcZK/xKc=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "e7439b6b14ad3cc35d05608ebca9bce01a25f5f8",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}