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/build.rs

@@ -0,0 +1,5 @@
+// generated by `sqlx migrate build-script`
+fn main() {
+    // trigger recompilation when a new migration is added
+    println!("cargo:rerun-if-changed=../migrations");
+}