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

packaging/nix/module.nix

@@ -0,0 +1,42 @@
+# NixOS module: installs the package and runs the mount daemon in every
+# graphical session, as the other packages' user service does.
+#
+#   services.proton-drive-linux.enable = true;
+self:
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  cfg = config.services.proton-drive-linux;
+in
+{
+  options.services.proton-drive-linux = {
+    enable = lib.mkEnableOption "the Proton Drive for Linux mount daemon, app and tray";
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = self.packages.${pkgs.stdenv.hostPlatform.system}.proton-drive-linux;
+      defaultText = lib.literalExpression "proton-drive-linux.packages.\${system}.proton-drive-linux";
+      description = "The package to use.";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    environment.systemPackages = [ cfg.package ];
+    # The unit ships in the package; this links it and pulls it into the
+    # graphical session, which is what `systemctl --user enable` would do.
+    systemd.packages = [ cfg.package ];
+    systemd.user.services.proton-drive = {
+      wantedBy = [ "graphical-session.target" ];
+      # The mount runs fusermount3 from PATH, and it has to be the setuid
+      # wrapper; the PATH NixOS gives a unit does not include it.
+      path = [ "/run/wrappers" ];
+    };
+    programs.fuse.enable = lib.mkDefault true;
+    # Session tokens are kept in the Secret Service.
+    services.gnome.gnome-keyring.enable = lib.mkDefault true;
+  };
+}