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/src/domain/mod.rs

@@ -0,0 +1,38 @@
+//! Domain layer - Pure business logic with no external dependencies
+//!
+//! This module contains the core business domain for the StA relay control system.
+//! It follows **Domain-Driven Design** principles with rich domain models and clear
+//! ubiquitous language.
+//!
+//! # Architecture Principles
+//!
+//! - **No external dependencies**: Domain layer depends only on Rust standard library
+//! - **Inward-pointing dependencies**: Infrastructure/Application depend on Domain, never reverse
+//! - **Rich domain models**: Entities and value objects encapsulate business rules
+//! - **Ubiquitous language**: Code reflects real-world relay control domain concepts
+//!
+//! # Planned Submodules
+//!
+//! - `relay`: Core relay domain (RelayId, RelayState, RelayLabel, Relay entity)
+//!   - Value objects with validation (newtypes following TyDD principles)
+//!   - Domain entities (Relay, RelayCollection)
+//!   - Repository traits (RelayLabelRepository)
+//!   - Controller traits (RelayController)
+//!   - Domain errors (DomainError, ValidationError)
+//!
+//! # Type-Driven Development (TyDD)
+//!
+//! Domain types follow "make illegal states unrepresentable" principle:
+//! - `RelayId`: Newtype wrapping u8, validated to 1..=8 range
+//! - `RelayLabel`: String wrapper, validated max 50 chars
+//! - `RelayState`: Enum (On, Off) - no invalid states possible
+//!
+//! See `specs/001-modbus-relay-control/types-design.md` for complete type design.
+//!
+//! # References
+//!
+//! - Architecture: `specs/constitution.md` - Hexagonal Architecture principles
+//! - Type design: `specs/001-modbus-relay-control/types-design.md`
+//! - Domain specification: `specs/001-modbus-relay-control/spec.md`
+
+pub mod relay;

backend/src/domain/relay/mod.rs

@@ -0,0 +1,9 @@
+//! Relay domain module.
+//!
+//! This module contains the core domain logic for relay control and management,
+//! including relay types, repository abstractions, and business rules.
+
+/// Repository trait and error types for relay persistence.
+pub mod repository;
+/// Domain types for relay identification and control.
+pub mod types;

backend/src/domain/relay/repository.rs

@@ -0,0 +1,15 @@
+use super::types::RelayId;
+
+/// Errors that can occur during repository operations.
+///
+/// This enum provides structured error handling for all data persistence
+/// operations related to relay management.
+#[derive(Debug, thiserror::Error)]
+pub enum RepositoryError {
+    /// A database operation failed with the given error message.
+    #[error("Database error: {0}")]
+    DatabaseError(String),
+    /// The requested relay was not found in the repository.
+    #[error("Relay not found: {0}")]
+    NotFound(RelayId),
+}

backend/src/domain/relay/types.rs

@@ -0,0 +1,14 @@
+/// Unique identifier for a relay in the system.
+///
+/// Uses the newtype pattern to provide type safety and prevent mixing relay IDs
+/// with other numeric values. Valid values range from 0-255, corresponding to
+/// individual relay channels in the Modbus controller.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+#[repr(transparent)]
+pub struct RelayId(u8);
+
+impl std::fmt::Display for RelayId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.0.fmt(f)
+    }
+}