build: 添加 Linux 多发行版打包能力

提供 Electron 目录包、统一安装树、deb、rpm、Arch/pacman 和 Nix flake 配置，并随包安装 desktop、图标、自启动项与 Fcitx5 addon。

AI-assisted-by: Codex
AI-model: gpt-5.6-sol
AI-reasoning-effort: medium
Co-authored-by: Codex <codex@openai.com>

flake.nix

@@ -0,0 +1,40 @@
+{
+  description = "MeloASR — web-backed voice input for Fcitx5";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs = { self, nixpkgs }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+    in {
+      packages = forAllSystems (system:
+        let pkgs = import nixpkgs { inherit system; };
+        in {
+          default = pkgs.callPackage ./packaging/nix/package.nix { };
+          meloasr = self.packages.${system}.default;
+        });
+
+      apps = forAllSystems (system: {
+        default = {
+          type = "app";
+          program = "${self.packages.${system}.default}/bin/meloasr";
+        };
+      });
+
+      devShells = forAllSystems (system:
+        let pkgs = import nixpkgs { inherit system; };
+        in {
+          default = pkgs.mkShell {
+            packages = with pkgs; [
+              nodejs
+              cmake
+              pkg-config
+              fcitx5
+              nlohmann_json
+            ];
+          };
+        });
+    };
+}
+