feat: add Nix flake with dev shell, package, and NixOS/Home Manager modules

Reproducible development environment for NixOS/Nix contributors:
- Dev shell with Node 22, system Electron, Playwright, LD_LIBRARY_PATH
  for X11/Wayland/audio libs, activated automatically via direnv
- buildNpmPackage derivation wrapping system Electron with desktop file
  and hicolor icons
- NixOS module (programs.openscreen.enable) with xdg-desktop-portal
- Home Manager module for per-user installation
- Overlay for composing with other flakes

Tested: nix flake show, nix develop, nix build, nixos-rebuild switch

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1775710090,
+        "narHash": "sha256-ar3rofg+awPB8QXDaFJhJ2jJhu+KqN/PRCXeyuXR76E=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "4c1018dae018162ec878d42fec712642d214fdfa",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}