Add the PDrive repository social preview

[ClaudiuSchuster]

Summary

• add a purpose-built 1280 x 640, distro-neutral repository social preview
• use the broader message “Native file-explorer access. Resilient uploads. Clear live insight.”
• preserve a Nemo-specific variant for a future Nemo software-store listing
• check in the lossless artwork source, canonical-icon composition, SVG overlay, and variant profiles

Reproducible workflow

• make social-preview renders every configured profile
• make check-social-preview fails when committed PNGs differ from a fresh render
• each profile owns its copy, badge, dimensions, and repository-relative output path
• future file-manager or distribution variants automatically join both commands through variants.json

Visual and privacy checks

• solid sRGB PNGs at 1280 x 640
• generic preview: 525 KB; Nemo preview: 525 KB
• visually checked at full size and repository-card size
• contains no screenshots, private pixels, or local user data
• abstract event-horizon background was generated for this asset; branding and text are overlaid deterministically

Verification

• portable repository suite: passed
• make check-social-preview: passed for both variants
• Super-Linter 8.7.0: passed
• git diff --check: passed
• optional GTK/Xvfb tests skipped locally because xvfb-run is unavailable

After merge, .github/social-preview.png still needs to be uploaded once in Repository settings -> Social preview. Keeping the issue open until that configuration step is complete.

Refs #8