chore(library): update routes, redirects, and naming (#48668)

Our UI Library registry is expanding to include blocks that go beyond UI
and in some cases focus purely on back-end. This PR is a precursor to
adding more back-end related blocks. This PR includes the `ui-library ->
library` rename plus redirects and small UI copy updates. Since this is
a rename we'll need to update Vercel configuration.

## Vercel rollout

Keep the Library project Root Directory as `apps/ui-library`

1. In the **Library** Vercel project, set:

   `NEXT_PUBLIC_BASE_PATH=/library`

Apply it to Preview and Production, then redeploy the Library project.

2. In the **www** Vercel project, add:

`NEXT_PUBLIC_LIBRARY_URL=<current value of NEXT_PUBLIC_UI_LIBRARY_URL>`

Apply it to Preview and Production. Keep `NEXT_PUBLIC_UI_LIBRARY_URL`
during the migration, then redeploy the www project.

3. Deploy in this order:

   1. Library project
   2. www project

4. Validate:

   - `/library`
   - `/library/docs/nextjs/password-based-auth`
   - `/ui` redirects to `/library`
- `/ui/docs/nextjs/password-based-auth` redirects to
`/library/docs/nextjs/password-based-auth`
- `/ui/docs/ai-editors-rules/*` still uses its existing Docs redirects

No Vercel dashboard redirect rules are needed. Environment-variable
changes require a new deployment.

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

* **New Features**
* Supabase UI Library has been renamed to **Supabase Library** across
navigation, pages, documentation, and resource links.
* The Library is now available at `/library`, with updated descriptions
covering components, blocks, and developer tools.
* **Bug Fixes**
* Added permanent redirects from legacy `/ui` URLs to corresponding
`/library` paths.
* Updated links throughout the site and documentation to prevent broken
navigation and references.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/learn/scripts/build-llms-txt.ts

@@ -1,7 +1,7 @@
 import fs from 'fs'
 import path from 'path'
 
-const BASE_URL = 'https://supabase.com/ui/docs'
+const BASE_URL = 'https://supabase.com/library/docs'
 
 interface DocMeta {
   title: string