fix(skin-market): add desktop and website links

[bobleer]

Summary

• Link the catalog and detail-page BitFun Desktop installation guidance to the official GitHub Releases page.
• Add the BitFun official website link to the Skin Market footer, matching the MiniApp Market behavior.
• Centralize and test both external destinations.
• Register the newly merged manual Skin submission form in the Appearance surface contract so the full Web UI build passes.

Type and Areas

Type: bug fix / UI/UX / test / CI

Areas: Skin Market web, Web UI Appearance contract

Motivation / Impact

Skin Market visitors can now reach the BitFun Desktop downloads directly from the installation guidance and navigate to the BitFun official website from the footer. The client manual-upload channel from #2017 also satisfies the Appearance contract audit used by CI.

Verification

• pnpm --dir src/skin-market-web run type-check — passed
• pnpm --dir src/skin-market-web run test — 5 files / 15 tests passed
• pnpm --dir src/skin-market-web run build — passed
• pnpm run appearance:contract-audit — passed
• pnpm run build:web — passed (includes Appearance audit, type-check, Vite production build, and Monaco asset verification)
• git diff --check — passed
• Deployed commit 31551b797515d99c8b315b04abeacb2b5ec3ad56 to market.openbitfun.com/skin; origin/public health and both external URLs in the served asset were verified.

Reviewer Notes

AI-assisted, fully tested. The manual Skin submission feature requested alongside this work is present in current main via #2017; this PR fixes its missing Appearance surface registration after the first CI run exposed it.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.