fix(portal-shell): use .postcssrc.json so tailwind utilities are emitted (#75)

## Summary

The portal-shell scaffold shipped a `postcss.config.js`, but `@angular/build` (Angular 17+ esbuild pipeline) does **not** load that file format — it only reads `.postcssrc.json`. Result: the `@tailwindcss/postcss` plugin was never registered, Tailwind ran with no source files visible, generated only its `@theme` block, and dropped every utility class on the floor.

The page therefore rendered unstyled — black text on white — even though `nx build` reported success and shipped a 23 KB `styles*.css`.

Rename / re-encode the config to `.postcssrc.json`. Same plugin, same options, just the file format Angular's esbuild adapter actually reads.

## Verification

| | Before | After |
| - | - | - |
| Class selectors in `dist/apps/portal-shell/browser/styles*.css` | **0** | **75** (production) |
| `.text-3xl`, `.bg-amber-50`, `.font-semibold` etc. emitted | ❌ | ✓ |
| Pages render with intended Tailwind layout | ❌ | ✓ |

```bash
pnpm exec nx build portal-shell --configuration=production
grep -oE '\.[a-zA-Z][a-zA-Z0-9_-]*' dist/apps/portal-shell/browser/styles*.css | sort -u | wc -l
# 75
```

## Doc update

`docs/development.md` repo-layout walkthrough now reads `.postcssrc.json` and explicitly calls out that `postcss.config.js` is ignored by `@angular/build` — so a future contributor reaching for the legacy filename gets a hint instead of a silent failure.

## Test plan

- [ ] CI green on this PR.
- [ ] Local: bring up the SPA dev server, `localhost:4200` shows the styled layout — header bar with brand link, system-status widget with rounded card, footer with the two language links.
- [ ] Production build: `nx build portal-shell --configuration=production` succeeds; `dist/.../styles*.css` contains tens of utility-class selectors (not just `@theme` tokens).

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #75

apps/portal-shell/.postcssrc.json

@@ -0,0 +1,5 @@
+{
+  "plugins": {
+    "@tailwindcss/postcss": {}
+  }
+}