Add offline dependency download scripts for air-gapped environment

- download-all-dependencies.sh: Downloads Qt6 6.10.2, source tarballs, and Ubuntu .deb packages
- download-deb-packages.sh: Downloads Ubuntu .deb packages in Docker container
- verify-offline-cache.sh: Verifies offline cache completeness
- offline-cache/README.md: Documentation for offline cache structure and usage
- .gitignore: Exclude external-dependencies/offline-cache/ from version control (2.3GB)

The offline cache must be transferred to air-gapped environment via secure media.
Excluded from git due to size and security best practices.

Co-authored-by: Copilot <2235566219+Copilot@users.noreply.github.com>
Entire-Checkpoint: 01M6PFZM0GHMBF46BKYPKZC192

.gitignore

@@ -88,4 +88,5 @@ fix_*.py
 rpms/
 deb_stage
 packages-out
+external-dependencies/offline-cache/