fix(ui-patterns): fix admonition self-import + add case-sensitivity guard (#48344)

Fixes the TanStack Studio app failing to load locally, and adds a CI
guard so the same class of bug can't come back.

`packages/ui-patterns/src/admonition.tsx` is a back-compat shim
containing `export * from './Admonition'`. On a case-insensitive
filesystem (macOS, Windows) the resolver tries `./Admonition.tsx` before
the directory index — and that's the same file. The shim re-exported
itself and exported nothing, so every consumer of
`ui-patterns/admonition` blew up with `does not provide an export named
'Admonition'`, plus knock-on Vite dep-optimizer errors about missing
chunks.

It works on Linux, so typecheck, lint, build and tests all pass on CI.
This only reproduces on dev machines.

**Changed:**

- `admonition.tsx` now points at `./Admonition/index` explicitly, so the
specifier can't resolve back to itself

**Added:**

- `scripts/check-case-hazards.mjs` — dependency-free, two textual checks
so they fire on Linux CI:
- **Self-resolving imports**: for `dir/X.tsx`, flags any extension-less
relative specifier resolving to `dir/X` case-insensitively
- **Case-colliding paths**: tracked paths (files and directory prefixes)
equal when lowercased, which can't coexist in a case-insensitive
checkout
- `pnpm test:case-hazards`, plus a step in `typecheck.yml` after
`setup-node` but before `pnpm install` — no deps needed, fails fast

Scoped check 1 to genuine self-imports rather than all case-insensitive
file/directory ambiguity. The broader rule lights up ~45 legitimate
routing pairs (`_app.tsx` + `_app/`, `changelog.tsx` + `changelog/`) and
would get switched off within a week. This version has zero false
positives on master today.

## Follow-up (not in this PR)

The underlying duplication is still there: `src/admonition.tsx` and
`src/Admonition/` both exist, and the ~14 internal `'../Admonition'`
imports inside the package resolve through the shim on macOS but through
the directory on Linux. Two resolution paths for one module is exactly
what produced this.

Real fix is to collapse it. Current counts: **246** files import
`ui-patterns/admonition`, **0** import `ui-patterns/Admonition`. So
either rename the directory to lowercase and delete the shim (zero
import churn, but one lowercase dir among ~50 PascalCase siblings), or
codemod the 246 imports to PascalCase to match the package convention.
I'd lean to the codemod, on a day it won't conflict with in-flight
branches.

## To test

- `pnpm test:case-hazards` on master → passes, ~16.5k files checked
- Revert `admonition.tsx` to `export * from './Admonition'` and re-run →
fails with the offending file and the suggested fix
- Confirm the fixed form `'./Admonition/index'` is *not* flagged
- With the fix in place: `rm -rf apps/studio/node_modules/.vite`, then
`STUDIO_FRAMEWORK=tanstack pnpm dev:studio` → app loads, no
`Pre-transform error` and no missing-export error in the console


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

## Summary by CodeRabbit

* **Bug Fixes**
* Improved detection of file-path casing issues that could cause
failures on case-insensitive systems.
* Corrected a module re-export to ensure the intended UI component is
exposed consistently.

* **Tests**
  * Added a dedicated case-sensitivity hazard check.
* Integrated the check into the type-check workflow for earlier issue
detection.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Alaister Young <10985857+alaister@users.noreply.github.com>

package.json

@@ -22,6 +22,7 @@
     "dev:design-system": "turbo run dev --filter=design-system --parallel",
     "lint": "turbo run lint",
     "typecheck": "turbo --continue typecheck",
+    "test:case-hazards": "node scripts/check-case-hazards.mjs",
     "test:prettier": "SORT_IMPORTS=false prettier --config prettier.config.mjs --cache --check '{apps,packages,blocks,examples,i18n}/**/*.{js,jsx,ts,tsx,css,md,mdx,json}'",
     "format": "SORT_IMPORTS=false prettier --config prettier.config.mjs --cache --write '{apps,packages,blocks,examples,i18n}/**/*.{js,jsx,ts,tsx,css,md,mdx,json}'",
     "test:docs": "turbo run test --filter=docs",