fix(portal-shell): same html/body overflow-y hidden shield as admin (#177)

## Summary

Mirror of #176 onto `portal-shell`. Same one-line shield, same rationale: the two apps share an identical `:host { height: 100vh }` + `<main> overflow-y: auto` layout, so the same defensive `html, body { overflow-y: hidden }` rule belongs on both surfaces. Brings the public-facing shell to the same posture as the admin one — any future layout escape stops at the shell boundary rather than producing a phantom body scrollbar plus an empty band below the footer.

## What lands

`apps/portal-shell/src/styles.css`:

```css
html,
body {
  overflow-y: hidden;
}
```

Same block as #176 with a comment that explicitly cross-references the admin shield so future contributors don't accidentally diverge the two apps.

## Notes for the reviewer

- **Why now, rather than waiting for portal-shell to demonstrate the symptom?** #176's reviewer note said this would land "if/when a layout escape shows up there." The user asked for parity immediately, on the reasoning that the shell contract is the *same* on both apps — the shield is defensive and one-line, so coupling the two posture changes is cheaper than tracking a "TODO once we see it in shell".
- **No new tests.** Same justification as #176 — the change is at the global stylesheet level, has no behavioural surface, and the manual repro path already exists (force a chart or wide element past the viewport; pre-shield → body scrollbar + footer gap; post-shield → clipped at the shell root, `<main>` still scrolls).
- **Element-level scrolling on `<main>` is unaffected.** The skip-link, sidebar, and footer all keep their pinned positions; long routes (the user list, future content pages) scroll inside `<main>` as designed.

## Test plan

- [x] `pnpm nx build portal-shell` — clean.
- [x] `pnpm nx test portal-shell` — green.
- [x] `pnpm nx lint portal-shell` — clean.
- [x] `pnpm nx run-many -t build test lint -p portal-shell,portal-admin` — green (admin and shell share the build cache; touching only one file invalidates only the shell target).
- [ ] **Manual smoke** — `pnpm nx serve portal-shell`:
  - Open `/fr` and `/en`, scroll long pages — `<main>` scrolls, body doesn't.
  - Resize across the breakpoint where the sidebar collapses — body still doesn't scroll; sidebar/footer pin correctly.
  - Toggle dark mode — no visual regression.

## What's next

Nothing pending on this shell-shield front. The two apps are now symmetric; if a third app appears (it won't in v1) the same pattern is documented in both `styles.css` headers.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #177

apps/portal-shell/src/styles.css

@@ -20,3 +20,18 @@
 
 /* Brand palette tokens — shared with `portal-admin` once it ships. */
 @import '../../../libs/shared/tokens/src/brand-tokens.css';
+
+/*
+ * The shell is a "fills the viewport, never scrolls the body" layout:
+ * <app-root> is locked at height: 100vh, and <main> owns its own
+ * overflow-y. If anything inside the shell ever pushes content past
+ * the viewport (a wide chart, a flex sizing bug, a third-party
+ * iframe), we'd rather clip than show a phantom body scrollbar plus
+ * the empty space below the footer that comes with it. The element-
+ * level overflow on <main> still produces a real, scrollable region
+ * for content that overflows by design. Same shield as portal-admin.
+ */
+html,
+body {
+  overflow-y: hidden;
+}