feat(library): serve the block catalog as Markdown and harden the exporter (#50370)

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

Feature, bug fix.

Part 3 of 6 in a stack that splits the library redesign into reviewable
pieces.

## What is the current behavior?

An agent can already fetch any guide as Markdown, but has no way to find
out what guides exist: the entry point is a rendered React page.

The exporter also fails quietly in ways that ship wrong output rather
than failing the build:

- An unknown component silently unwraps to its children, so a component
rename drops its rendered content.
- A registry item that cannot be read produces a page with no file
listing.
- A link to a missing page produces a 404 URL.
- An unrecognized install framework produces a plausible command for the
wrong CLI.
- Only absolute `/library/docs` links are rewritten, so in-page anchors
and sibling links break in the export.

## What is the new behavior?

`/library` negotiates Markdown the same way the guides do — `Accept:
text/markdown`, or an explicit `/library/index.md` — and returns a
categorized catalog with every block, its description, its framework
variants, and a link to each guide's Markdown.

`config/library.ts` is the single catalog description the generator
reads, and a test ties it to the content directory in both directions: a
guide cannot be added without a catalog entry, or listed without a
guide.

Each quiet failure above now throws, and links resolve against the page
they appear on and are checked against the set of published documents.

```bash
curl -H 'Accept: text/markdown' https://supabase.com/library
```

## Additional context

`config/library.ts` also carries the category and preview metadata the
redesigned homepage consumes in the last PR of the stack; here it is
exercised by the Markdown index and its test.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Added a browsable library catalog covering categories, blocks, starter
apps, and supported frameworks.
- Added Markdown versions of the library homepage and documentation for
compatible tools and workflows.
- Added framework-aware links and expanded registry information,
including dependencies and source details.
- Markdown requests now work for the homepage and documentation, while
browser requests continue receiving HTML.

- **Bug Fixes**
- Improved document link handling, metadata validation, slug
consistency, and detection of duplicate or missing documentation
entries.

- **Tests**
- Added coverage for catalog routes, Markdown generation, homepage
negotiation, document parsing, and framework-specific links.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Ivan Vasilov <vasilov.ivan@gmail.com>

apps/ui-library/package.json

@@ -7,20 +7,20 @@
     "preinstall": "npx only-allow pnpm",
     "dev:content": "velite dev",
     "dev:next": "next dev --port 3004",
-    "dev": "run-p build:content build:registry build:markdown && run-p --race dev:*",
+    "dev": "pnpm build:registry && run-p build:content build:markdown build:markdown-index && run-p --race dev:*",
     "build:content": "velite build --strict",
     "build:registry": "rimraf -G public/r/* && tsx ./scripts/build-registry.mts && shadcn build public/r/registry.json && tsx scripts/clean-registry.ts",
     "build:markdown": "tsx ./scripts/build-markdown.ts",
-    "build:llms": "tsx ./scripts/build-llms-txt.ts",
+    "build:markdown-index": "tsx ./scripts/build-markdown-index.ts",
     "build:next": "next build --turbopack",
-    "build": "run-p build:content build:registry build:llms build:markdown && pnpm build:next",
+    "build": "pnpm build:registry && run-p build:content build:markdown build:markdown-index && pnpm build:next",
     "test": "pnpm build:markdown && vitest",
     "test:headless-tools": "tsx scripts/test-headless-tools.mts",
     "start": "next start",
     "lint": "eslint .",
     "lint:mdx": "supa-mdx-lint content --config ../../supa-mdx-lint.config.toml",
     "clean": "rimraf .next .turbo tsconfig.tsbuildinfo .contentlayer .velite",
-    "typecheck": "run-p build:content build:markdown && tsc --noEmit -p tsconfig.json"
+    "typecheck": "run-p build:content build:markdown && next typegen && tsc --noEmit -p tsconfig.json"
   },
   "dependencies": {
     "@hookform/resolvers": "^3.1.1",