feat(server): highlight the Rust rewrite and demo on the server landing page

[everpcpc]

Why

The /server/ landing page never mentioned that KMServer is a Rust reimplementation, had no link to the live demo, and its "Why KMServer?" section was too generic to sell the advantages.

What changed

• Hero: subtitle now reads "Komga reimplemented in Rust: one static binary, a bounded footprint, and the web UI included."; "Written in Rust" added as the first chip; page title and meta description mention Rust.
• New "Try it in your browser." section right after the hero, with the demo.kmworks.date credentials and an "Open the demo" button (same layout as the homepage demo section).
• "Why KMServer?" rebuilt around the Rust advantages, now four cards: Written in Rust, Bounded memory (16 MiB SQLite page-cache budget per database, capped allocator arenas — facts from the enhancements docs), Runs anywhere, Web UI included.
• "Technical details" renamed to "Beyond compatibility" with the subtitle "Capabilities the Java version does not have." and a chip row linking to each enhancement doc page (webhooks, history events, EPUB 2 metadata, komf, file-backed thumbnails, next/previous, reading stats, smart lists, server stats).
• The demo credential styles moved from the homepage's scoped styles into src/styles/landing.css so both landing pages share them; the homepage is visually unchanged.

No performance numbers are claimed — none are published in the docs yet.

Verification

• pnpm typecheck && pnpm build && pnpm exec nimbus-docs check && pnpm lint:docs all pass.
• Visually checked /server/ (hero, demo, Why KMServer, Beyond compatibility) and the homepage demo section in a local preview.