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/.tarpaulin.local.toml

@@ -0,0 +1,7 @@
+[all]
+out = ["Html", "Lcov"]
+skip-clean = true
+target-dir = "coverage"
+output-dir = "coverage"
+fail-under = 60
+exclude-files = ["target/*", "private/*"]