refactor: reorganize project into monorepo with frontend scaffolding

Convert project from single backend to monorepo structure with separate
frontend (Vue 3 + TypeScript + Vite) and backend directories. Updates
all configuration files and build system to support both workspaces.

Ref: T007 (specs/001-modbus-relay-control)

nix/shell.nix

@@ -11,6 +11,7 @@ inputs.devenv.lib.mkShell {
   modules = [
     {
       packages = with pkgs; [
+        # Backend
         (rustVersion.override {
           extensions = [
             "clippy"
@@ -28,6 +29,13 @@ inputs.devenv.lib.mkShell {
         marksman # Markdown LSP server
         sqlx-cli
         tombi # TOML LSP server
+
+        # Frontend
+        nodejs_24
+        rustywind # tailwind
+        nodePackages.prettier
+        nodePackages.eslint
+        nodePackages.pnpm
       ];
 
       processes.run.exec = "bacon run";