feat(server): implement basic backend API

- Added basic Express server setup in `src/server/main.ts`.
- Configured `dotenv` to load environment variables.
- Implemented in-memory storage for session state (pending, success, failed).
- Created `/api/digiid/start` endpoint:
    - Generates session ID and nonce.
    - Constructs callback URL from `PUBLIC_URL`.
    - Determines `unsecure` flag based on URL scheme.
    - Stores initial session state.
    - Generates QR code data URL.
    - Returns `sessionId` and `qrCodeDataUrl`.
    - Includes placeholder for `digiidTs.generateDigiIDUri`.
- Created `/api/digiid/callback` endpoint:
    - Receives address, uri, signature from DigiID app.
    - Parses nonce from received URI.
    - Looks up session by nonce.
    - Reconstructs expected callback URL.
    - Updates session state based on placeholder verification.
    - Includes placeholder for `digiidTs.verifyDigiIDCallback`.
    - Responds 200 OK as per DigiID protocol.
- Created `/api/digiid/status/:sessionId` endpoint:
    - Retrieves and returns session status, address, and error.
- Added address type helper `getDigiByteAddressType` in `src/server/utils.ts` (with placeholder logic for DGA).
- Added `dev:backend` script to `package.json` using `nodemon` and `ts-node/esm`.
- Added `"type": "module"` to `package.json`.
- Installed `@types/dotenv`.
- (Note: Outstanding TypeScript linter errors related to async Express handlers require further investigation).

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