chore: move biome config to biome.jsonc and baseline existing violations

biome.json is strict JSON; the jsonc extension allows the annotations
on the baseline entries. Rules that currently fail are downgraded to
warn with a note against issue #421 — restore each to error as its
occurrences are fixed. files.experimentalScannerIgnores keeps the
scanner out of .worktrees/ so a local worktree's own config can't
conflict with the root one.

Also fixes the handful of auto-fixable errors (unused imports in docs/
and landing/, @ts-expect-error over @ts-ignore). bun run lint is now
green: 0 errors, 96 warnings visible as debt.

docs/app/[[...slug]]/page.tsx

@@ -3,7 +3,6 @@ import { DocsBody, DocsDescription, DocsPage, DocsTitle } from 'fumadocs-ui/page
 import type { Metadata } from 'next';
 import { notFound } from 'next/navigation';
 import { MarkdownCopyButton, ViewOptionsPopover } from '@/components/ai/page-actions';
-import { APIPage } from '@/components/api-page';
 import { getPageImage, source } from '@/lib/source';
 import { getMDXComponents } from '@/mdx-components';