feat(theme): redesign the homepage around the mascot and the projects

[16bit-ykiko]

The homepage is the entry for every clice.io project. It should say at a glance what the group builds and show the projects, and it shouldn't repeat the same links. It also replaces the remaining AI-looking homepage art.

Hero

• The mascot is redrawn standing quietly with her hands clasped, and the bow keeps its gold C clasp.
• She now stands on the right, so she no longer sits right under the navbar logo.
• The headline and the introduction share one speech balloon. Its tail is computed from the layout to reach her mouth, and stops at her outline.
• The introduction says what the tools do. There is one button, "Get started with clice".

Projects

• One manga page replaces the three cards: catter, clice and kotatsu are panels with slanted gutters, drawn from the grid layout at runtime.
• An arrow labelled compile_commands.json crosses from catter to clice, and two arrows rise from kotatsu into both.
• Each panel has a narration box, a line-icon portrait and a one-sentence summary. Each panel links to its docs; kotatsu has no docs, so it links to GitHub.
• The old project illustrations are gone.

Removed because they repeated the navbar and footer:

• the editors/platforms chips
• the Q&A strip
• the "Get involved" cards
• the per-card GitHub links

Body font

• M PLUS Rounded 1c only has Japanese kanji. In Chinese paragraphs about two thirds of the glyphs came from it and the rest (such as 构, 们) from Noto Sans SC, with Japanese punctuation mixed in.
• Body text is now Nunito for Latin and Noto Sans SC for all Chinese. Headings are unchanged.
• @fontsource/m-plus-rounded-1c and the never-imported @fontsource/kosugi-maru are dropped.

Other

• The footer shows the new logo instead of the old avatar.
• Images nothing references any more are deleted: the Q&A faces, the old avatar, hero and project art, the bow-logo variants, the wordmarks and image.png.
• The new hero's source image, prompt and build script are in art/home/.
• art/logo/build.sh now also writes favicon.ico, which some clients request regardless of the <link> tags.

Verified with vitepress build and vitepress preview. I loaded /, /zh/, /clice/, /zh/catter/, /blog, a guide page, a feature page and a 404 in light at 1440 px and dark at 390 px. None had failed requests, broken images, page errors or horizontal overflow. The homepage was also checked at 800 and 1600 px in vitepress dev.