feat: send confirmation email to sender

When users submit a contact form, they now receive a confirmation
email acknowlledging receipt of their message. The backend also
continues to send a notification email to the configured recipient.

If the backend fails to send the acknowledgement email to the sender,
it will assume the email is not valid and will therefore not transmit
the contact request to the configured recipient.

Changes:
- Refactor `send_email()` to `send_emails()` that sends two emails:
  - Confirmation email from the submitter
  - Notification email to the configured recipient
- Add `From<T>` implementations of various errors for new error type
  `ContactError`.
- Errors now return a translation identifier for the frontend.

README.md

@@ -1,3 +1,8 @@
+---
+include_toc: true
+gitea: none
+---
+
 # phundrak.com Backend
 
 The backend for [phundrak.com](https://phundrak.com), built with Rust and the [Poem](https://github.com/poem-web/poem) web framework.
@@ -178,6 +183,7 @@ just coverage
 - Tests use `get_test_app()` helper for consistent test setup
 - Telemetry is automatically disabled during tests
 - Tests are organized in `#[cfg(test)]` modules within each file
+- Email sending is tested using lettre's `StubTransport` for mocking SMTP operations
 
 ## Code Quality
 
@@ -256,12 +262,15 @@ backend/
 │   ├── startup.rs       # Application builder, server setup
 │   ├── settings.rs      # Configuration management
 │   ├── telemetry.rs     # Logging and tracing setup
+│   ├── errors.rs        # Error type re-exports
 │   ├── middleware/      # Custom middleware
 │   │   ├── mod.rs       # Middleware module
 │   │   └── rate_limit.rs # Rate limiting middleware
 │   └── route/           # API route handlers
 │       ├── mod.rs       # Route organization
-│       ├── contact.rs   # Contact form endpoint
+│       ├── contact/     # Contact form module
+│       │   ├── mod.rs   # Contact form endpoint
+│       │   └── errors.rs # Contact form error types
 │       ├── health.rs    # Health check endpoint
 │       └── meta.rs      # Metadata endpoint
 ├── settings/            # Configuration files
@@ -417,7 +426,7 @@ The workflow requires these GitHub secrets:
 - `DOCKER_PASSWORD` - Registry password or token
 - `CACHIX_AUTH_TOKEN` - (Optional) For Nix build caching
 
-See [.github/workflows/README.md](../.github/workflows/README.md) for detailed setup instructions.
+See [.github/workflows/README.md](./.github/workflows/README.md) for detailed setup instructions.
 
 ## License