fix: replace generic Error with proper NestJS HTTP exceptions in controllers

[Mustafa-Shoukat1]

Summary

Multiple controllers (channel, contact, group, label) throw generic JavaScript Error instead of NestJS HTTP exceptions (BadRequestException, NotFoundException). This causes 500 Internal Server Error responses instead of the correct 400/404 status codes that are already documented in the Swagger API specs.

Problem

When a session is not started (e.g., after a phone disconnects per #100), calling any endpoint on these controllers returns:

{
  \"statusCode\": 500,
  \"message\": \"Internal server error\"
}

Instead of the expected:

{
  \"statusCode\": 400,
  \"message\": \"Session is not started\"
}

Similarly, when a resource (channel, contact, group, label) is not found, the API returns 500 instead of 404.

This is inconsistent with:\n- The Swagger @ApiResponse decorators that document 400/404 responses\n- Other controllers/services in the codebase that correctly use NestJS exceptions (e.g., session.service.ts, status.service.ts, webhook.service.ts)

Changes

Backend (4 controllers)

| File | Fix |\n|------|-----|\n| src/modules/channel/channel.controller.ts | throw new Error(...) → BadRequestException / NotFoundException |\n| src/modules/contact/contact.controller.ts | throw new Error(...) → BadRequestException / NotFoundException |\n| src/modules/group/group.controller.ts | throw new Error(...) → BadRequestException / NotFoundException |\n| src/modules/label/label.controller.ts | throw new Error(...) → BadRequestException / NotFoundException |\n\n### Dashboard\n\n| File | Fix |\n|------|-----|\n| dashboard/src/services/api.ts | Added missing 'failed' to Session.status union type to match backend SessionStatus enum |\n\n## Related Issues\n\n- Partially addresses #100 — after a session disconnects from the phone, API calls now return proper 400 responses instead of 500\n\n## Testing\n\n- No existing unit tests for these controllers to break\n- Changes are minimal and mechanical (import swap + class swap)\n- Consistent with patterns already used in status.service.ts, webhook.service.ts, session.service.ts"}