build: fix Nix flake with correct npmDepsHash and working builds

- Copy working src/frontend code into packages/frontend/src
- Add vite.config.js and index.html to packages/frontend
- Add real backend WebSocket server source
- Switch flake buildPhase from pnpm to npm
- Force-add npm lockfiles (gitignored) for Nix source inclusion
- Set correct npmDepsHash for both frontend and backend packages

flake.nix

@@ -25,7 +25,7 @@
 
           src = ./packages/frontend;
 
-          npmDepsHash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
+          npmDepsHash = "sha256-yZajnZGRNnZo+zm8b0N9+nNOoyxt2EOoxwWYINl6FZU=";
 
           nativeBuildInputs = with pkgs; [
             nodejs_22
@@ -34,7 +34,7 @@
 
           buildPhase = ''
             runHook preBuild
-            corepack pnpm run build
+            npm run build
             runHook postBuild
           '';
 
@@ -59,7 +59,7 @@
 
           src = ./packages/backend;
 
-          npmDepsHash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
+          npmDepsHash = "sha256-GB3oOOAD7RJwkUR21zA9R66b5MlLXMU0IwOOgflfE14=";
 
           nativeBuildInputs = with pkgs; [
             nodejs_22
@@ -68,7 +68,7 @@
 
           buildPhase = ''
             runHook preBuild
-            corepack pnpm run build
+            npm run build
             runHook postBuild
           '';