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)

justfile

@@ -35,11 +35,11 @@ test:
 
 coverage:
 	mkdir -p coverage
-	cargo tarpaulin --config .tarpaulin.local.toml
+	cargo tarpaulin --config backend/.tarpaulin.local.toml
 
 coverage-ci:
 	mkdir -p coverage
-	cargo tarpaulin --config .tarpaulin.ci.toml
+	cargo tarpaulin --config backend/.tarpaulin.ci.toml
 
 check-all: format-check lint coverage audit