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/base.yaml

@@ -0,0 +1,17 @@
+application:
+  port: 3100
+  version: "0.1.0"
+
+rate_limit:
+  enabled: true
+  burst_size: 10
+  per_seconds: 60
+
+modbus:
+  host: "192.168.0.200"
+  port: 502
+  slave_id: 0
+  timeout_secs: 5
+
+relay:
+  label_max_length: 8

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"

backend/settings/production.yaml

@@ -0,0 +1,8 @@
+debug: false
+frontend_url: ""
+
+application:
+  name: "sta-prod"
+  protocol: https
+  host: 0.0.0.0
+  base_url: ""