feat: link local digiid-ts library and integrate into backend

- Added local `digiid-ts` library as a file dependency in `package.json`.
- Moved runtime dependencies (react, express, etc.) to `dependencies` section.
- Updated `src/server/main.ts`:
    - Imported `generateDigiIDUri`, `verifyDigiIDCallback`, and `DigiIDError` from `digiid-ts`.
    - Replaced placeholder URI generation with `generateDigiIDUri`.
    - Replaced placeholder callback verification with `verifyDigiIDCallback`.
    - Refactored `/api/digiid/callback` handler to use `try...catch` for verification, handling thrown `DigiIDError` on failure as per library design.
    - Ensured session state is updated and 200 OK response sent in `finally` block.
- Cleaned up related placeholder comments in `src/server/main.ts`.
- (Requires `npm install` to be run after checkout)

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