[Task] Apply the TrustDB brand mark to repository entry points

[ryan-wong-coder]

Background

The desktop application already contains the approved TrustDB check-mark icon, but the repository and organization entry points still use generic GitHub presentation.

Scope

• Use clients/desktop/build/appicon.png as the canonical TrustDB brand mark.
• Display the mark in both repository READMEs.
• Add a reusable brand-assets note and a 1280 by 640 repository social preview.
• Configure the GitHub repository social preview and WowTrust organization avatar.
• Improve the repository and organization presentation without introducing a second logo.

Acceptance criteria

• The existing icon remains the source of truth.
• English and Chinese README render the same mark at an appropriate size.
• Social preview uses the same colors and icon and meets GitHub dimensions.
• Repository settings show the new social preview.
• Organization avatar uses the approved mark.
• Unrelated local assets are not committed.

Validation

• Inspect generated image dimensions and visual output.
• git diff --check
• Verify GitHub settings after upload.