fix(docs): drop tabMode="top" that hid article body

[pratyush618]

Summary

Commit 1b2bb18 added tabMode="top" to docs/src/app/docs/layout.tsx. In Fumadocs UI 16.8.5 this renders the auto-generated tab strip into [grid-area:main] — the same CSS grid cell as the article — with z-10 and an opaque bg-fd-background. The strip stretches across the full main cell and completely covers <DocsTitle> plus the MDX body. The sidebar tree and right-side TOC still render, which is exactly what the bug report shows: a fully-blank center column on every docs page.

This was reproducible both locally (pnpm dev) and on docs.byteveda.org/taskito/.... Confirmed via grep '\[grid-area:main\]' out/docs/getting-started/installation.html returning 2 elements before, 1 after.

The fix drops the prop so Fumadocs falls back to the default tabMode="auto", which renders the same auto-generated section list as a SidebarTabsDropdown inside the sidebar header — which is exactly what 1b2bb18's message described ("promote top sections to sidebar tabs").

• One-line change in docs/src/app/docs/layout.tsx.
• root: true flags on the section meta.json files are still consumed by tabMode="auto", so the section dropdown is populated unchanged.

Test plan

• cd docs && pnpm types:check — clean
• cd docs && pnpm lint — clean
• cd docs && pnpm build — succeeded
• Built HTML for getting-started/installation, architecture/overview, guides/core/tasks, architecture, api-reference each contains exactly one [grid-area:main] element (the <article id="nd-page">)
• Article body, h1 "Installation", description, callout, code blocks all present in built HTML
• Sidebar header now contains the auto-generated section dropdown button ("Getting Started" with up/down chevron)
• After merge, verify on docs.byteveda.org/taskito/docs/getting-started/installation that the article renders with title, description, and content visible