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/application/mod.rs

@@ -0,0 +1,60 @@
+//! Application layer - Use cases and orchestration logic
+//!
+//! This module implements business use cases by coordinating domain entities and
+//! infrastructure services. It contains the application logic that orchestrates
+//! domain behavior without implementing domain rules directly.
+//!
+//! # Architecture Principles
+//!
+//! - **Depends on Domain layer**: Uses domain entities, value objects, and traits
+//! - **Framework-independent**: No dependencies on HTTP, database, or other infrastructure
+//! - **Use case driven**: Each module represents a specific business use case
+//! - **Testable in isolation**: Can be tested with mock infrastructure implementations
+//!
+//! # Planned Submodules
+//!
+//! - `relay`: Relay control use cases
+//!   - `get_status`: Retrieve current state of one or all relays
+//!   - `toggle_relay`: Switch relay on/off with validation
+//!   - `bulk_control`: Control multiple relays (all on, all off, pattern)
+//!   - `update_label`: Manage relay labels with persistence
+//!   - `get_health`: Check device health and connectivity
+//!
+//! # Use Case Pattern
+//!
+//! Each use case follows this pattern:
+//! 1. Accept domain types as input (validated at boundary)
+//! 2. Orchestrate domain entities and services
+//! 3. Return domain types or application-specific results
+//! 4. Depend on traits (RelayController, RelayLabelRepository), not concrete types
+//!
+//! # Example Use Case Structure
+//!
+//! ```rust,ignore
+//! pub struct ToggleRelay {
+//!     controller: Arc<dyn RelayController>,
+//!     repository: Arc<dyn RelayLabelRepository>,
+//! }
+//!
+//! impl ToggleRelay {
+//!     pub async fn execute(&self, relay_id: RelayId) -> Result<Relay, ApplicationError> {
+//!         // 1. Read current state
+//!         let current = self.controller.read_relay_state(relay_id).await?;
+//!
+//!         // 2. Toggle state (domain logic)
+//!         let new_state = current.toggle();
+//!
+//!         // 3. Write new state
+//!         self.controller.write_relay_state(relay_id, new_state).await?;
+//!
+//!         // 4. Return updated relay
+//!         Ok(Relay::new(relay_id, new_state))
+//!     }
+//! }
+//! ```
+//!
+//! # References
+//!
+//! - Architecture: `specs/constitution.md` - Hexagonal Architecture principles
+//! - Use cases: `specs/001-modbus-relay-control/plan.md` - Implementation plan
+//! - Domain types: [`crate::domain`] - Domain entities and value objects