feat(docs-next): polish landing page (phase 5)

[pratyush618]

Phase 5 of the docs migration. Replaces the Phase 1 stub home page (src/app/(home)/page.tsx) with a polished landing.

Changes

• Hero — version pill (v0.12 — Rust core, native async, DAG workflows), bigger headline, code-preview pane with traffic-light header showing the minimal taskito example, three CTAs (Quickstart / Install / GitHub).
• Feature grid — 6 cards with lucide icons covering brokerless / Rust-powered / async-first / DAG workflows / resource system / production-ready. Each card has an accent-colored icon, hover affordance, and a tight 2-3 sentence pitch.
• Comparison section — new client component comparison.tsx with tabs for taskito vs Celery+Redis. Side-by-side install commands, services-to-run summary, and the same email task implemented in each stack.
• CTA card — closing rounded card pointing at the quickstart and full comparison page.
• Lucide icon fix — Github doesn't exist in lucide-react@1.14.0 (the lucide team removed it for trademark reasons); replaced with text-only "GitHub →" link.
• Lint clean — biome auto-fixed import ordering, JSON formatting, template literals, and JSX whitespace. Two pre-existing scaffold-CSS !important warnings remain (intentional, they're for body-scroll-locked).

What's NOT in here (deferred)

• Animated typing for the hero code preview — value/effort isn't there for v0.12. Static is fine.
• shadcn/ui setup — Tailwind primitives + lucide cover the landing page well enough; not worth the dependency surface. If we add a design system later it would benefit docs/ MDX components more than this single page.
• Taskito logo SVG — placeholder still uses the Fumadocs scaffold logo. Logo work is separate.

Test plan

• pnpm types:check — clean
• pnpm lint — exit 0 (2 known scaffold-CSS warnings, no errors)
• pnpm build — clean, all routes generate
• CI: Build Docs (Next) job stays green
• Manual smoke once merged: open dev server, switch tabs in the comparison, verify CTAs all resolve, hover affordances on feature cards work, dark + light mode both look good

After this lands, only Phase 7 (cutover — repoint DNS, decommission Zensical, remove docs/ and zensical.toml) remains.