Add support for gzip-compressed NZB files (.nzb.gz)

[javi11]

Summary

This PR extends NZB file support to include gzip-compressed files (.nzb.gz) in addition to standard .nzb files across the application. A new utility function HasNzbExtension() was introduced to centralize and standardize NZB file extension validation.

Key Changes

• New utility function: Added HasNzbExtension() in internal/importer/utils/nzbtrim/nzb_trim.go to check for both .nzb and .nzb.gz extensions (case-insensitive)
• Backend validation updates: Replaced inline extension checks with the new utility function in:
  • internal/api/sabnzbd_handlers.go (file upload and URL download handlers)
  • internal/api/queue_handlers.go (queue upload handler)
  • internal/api/nzb_stremio_handlers.go (Stremio NZB handler)
  • internal/importer/scanner/directory.go (directory scanner)
  • internal/importer/scanner/watcher.go (directory watcher)
• Frontend updates:
  • Updated file validation logic in frontend/src/components/queue/ImportMethods.tsx to accept both .nzb and .nzb.gz files
  • Updated file input accept attribute to include .nzb.gz
  • Updated user-facing error messages to reflect support for both formats

Implementation Details

• The HasNzbExtension() function performs case-insensitive matching by converting filenames to lowercase before checking suffixes
• All error messages have been updated to indicate support for both .nzb and .nzb.gz formats
• The change is backward compatible and doesn't affect existing .nzb file handling

https://claude.ai/code/session_01JrGP2iwN5ekVfdjpbfRJja