docs: use the new Ä mark in the README header

[jackwener]

Summary

Both READMEs still showed apps/desktop/assets/icon.png (the character mascot) next to the title. The icon set that landed in #3431 is the geometric Ä mark under apps/desktop/assets/app-icons/. Point the header at sky.png.

Hero banners are unchanged. Packaging/window default is still assets/icon.png; this PR is docs-only.

Test plan

• GitHub renders app-icons/sky.png in README.md and README.zh-CN.md
• Character mascot no longer appears in the README title row

[Astro-Han]

Reviewed at exact head da392974df59110fb16516fc6e06f8ced760665e. test is terminal green on this head.

Verified the substance rather than just the diff shape:

• apps/desktop/assets/app-icons/sky.png exists at this head (blob 7e3ee3dd), so neither README will render a broken image.
• Both paths are repo-root relative and match the surrounding markup, unchanged apart from the filename.
• Packaging is untouched: electron-builder.config.mjs, app-icon.ts, settings.ts, and verify-packaged-app.test.mjs all still reference assets/icon.png, so the shipped app icon is unaffected. The description's claim that this is documentation-only holds.

Approving. No P0–P2.

[P3] Provenance record now points at the icon that is no longer shown

docs/code-origin-audit.md:182 lists apps/desktop/assets/icon.png among the assets with recorded origin (ChatGPT Image, prompts not retained). Nothing under apps/desktop/assets/app-icons/ appears in that document.

So after this change the README's headline image is an asset whose origin is not recorded in the audit file, while the recorded one is no longer displayed there. Nothing here is wrong today — the audit file is about repository contents, not about which file the README happens to reference — but since that document exists precisely to answer "where did this artwork come from", the icon that represents the project on its front page is the one most worth having listed. Worth adding a line for the app-icons/ set when someone next touches that file; no change requested here.