Add portable Arch packaging for 0.8.0

[ClaudiuSchuster]

Summary

• centralize reviewed Debian-family and Arch Linux platform detection and package maps
• add relocatable user/system installation layouts plus a checksum-pinned native Arch package gate
• add privacy-safe real-desktop promotion reports, efficient distribution CI and the shared 0.8.0 version
• clamp long Control Center pages to the desktop work area after the clean Arch gate exposed a 1280x800 panel overlap

Support status

Linux Mint 22.x Cinnamon remains the supported reference platform. The Arch Linux 0.8.0-1 candidate has completed its clean Cinnamon/X11 promotion record; stable package publication follows the merged release. Ubuntu 24.04 is an active portability target; native Debian packaging follows separately.

The pull-request compatibility job reuses one Arch image for the runtime smoke and package gate, then retains the commit-bound compressed package for seven days. This is a test artifact, not a stable package publication.

Verification

• git diff --check
• make check
• real Cinnamon/X11 setup-wizard and GTK widget suites
• configured Mint desktop report: 20/20 automated gates passed
• Arch Linux runtime smoke
• Ubuntu 24.04 runtime smoke
• native Arch makepkg, namcap, archive, install and dispatcher probes
• Super-Linter 8.7.0
• clean Arch desktop installation, tray, file-manager and login lifecycle record
• post-fix remote artifact reinstalled in the clean Arch VM: 105 package files, 0 altered; preflight 15/15

The final PR artifact for head 418502c is proton-drive-linux-0.8.0-1-any.pkg.tar.zst with SHA-256 51291612104353a8b8f0f3a24c5d648193983300151baf7b010747366df6319e. The installed UI source matches the reviewed commit, and its visible content remains inside the 1280x760 Cinnamon work area.

Promotion rule

Publish v0.8.0 only after this PR is squash-merged and every required GitHub check is green on the resulting main commit.