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)

backend/settings/development.yaml

@@ -0,0 +1,8 @@
+frontend_url: http://localhost:3000
+debug: true
+
+application:
+  protocol: http
+  host: 127.0.0.1
+  base_url: http://127.0.0.1:3100
+  name: "sta-dev"