ci: install from the pinned image package indexes (#247)

Keep dependency resolution tied to the authenticated indexes in the official image. Current KDE Release metadata returns 502/504 while indexed package downloads work. Fresh-container dependency installation, strict make check with no skips, and make package all pass. Update contributor documentation and Unreleased; applet behavior and TODO are unchanged.
6106ee32d4Simone committed on 9/12/2026, 3:14:00 PM· committed by GitHubparent500c0f7
4 files changedLine totals unavailable