feat(portal-shell): app-shell layout with collapsable sidebar (#83)

## Summary

- Replace the flat header+main+footer layout with a real app-shell: fixed header on top, collapsable sidebar + scrollable main below. Sidebar state (collapsed / expanded) persists across reloads via `localStorage`.
- Introduce the APF brand palette as Tailwind v4 `@theme` tokens (primary teal `#12546c`, accent orange `#f7a919`) so every utility (`bg-brand-primary-500`, `text-brand-accent-400`, `ring-brand-primary-200`, …) is available from now on.
- Add an `<app-icon>` façade backed by `lucide-angular` for v1. Logical kebab-case names already match the icomoon-sprite convention, so the future migration is a single-file change in `icon.ts` and consumers stay untouched.
- Migrate the accessibility-statement links from the (now-deleted) footer to the bottom of the sidebar.

## Decisions worth flagging

- **Static menu, permission-shaped data.** Items point to `#` placeholders in v1, except *Dashboard* which is `routerLink="/"` so the active-state styling is visible on the home page. The `MenuItem` shape already carries an optional `requiredPermissions: string[]` so the permission-aware filter (PR 2, alongside ADR-0009 auth) plugs in without restructuring.
- **`<app-icon>` over direct lucide imports.** Consumers write `<app-icon name="bell">` rather than importing the lucide pascal-case symbol. When the icomoon sprite lands, only the registry in `icon.ts` changes — templates do not.
- **Sidebar persistence via `localStorage`, not backend.** Zero round-trip, survives reloads, falls back gracefully when storage is blocked (private mode). Eventually mirrored server-side if the user-preferences feature lands.
- **Footer removed entirely.** With the sidebar carrying the FR + EN accessibility-statement links and the role badge, the bottom rail no longer earned its vertical real estate. The version badge moved out for now; it will return as part of a debug/help menu when there's a real release to surface.

## Accessibility (ADR-0016)

- Skip-link preserved (WCAG 2.4.1 *Bypass Blocks*) and restyled in the brand palette.
- Sidebar exposes named landmarks (`<nav aria-label="Sections">`, `<nav aria-label="Accessibility">`) and the collapse button uses `aria-expanded` + a descriptive `aria-label`.
- Active links carry `ariaCurrentWhenActive="page"`.
- All interactive controls (header action buttons, sidebar links/toggle) meet the 44×44 px minimum hit-target.
- Sidebar width transition is skipped under `prefers-reduced-motion: reduce`.
- Lucide SVGs are marked `aria-hidden` (decorative); accessible names live on the parent control.

## Perf (ADR-0017)

- Production build: **100 kB gzip** initial transfer (budget: 300 kB). Lucide imports are tree-shaken — only the ~18 icons actually used ship.

## What this PR explicitly does NOT do

- Wire icon-set migration to icomoon (kept as a deferred swap behind `<app-icon>`).
- Filter the menu by permission (deferred to PR 2 once the auth flow lands).
- Replace the avatar placeholder with a real user menu (waits on ADR-0009).
- Implement the search input behavior (placeholder only; needs a search backend).

## Test plan

- [x] `pnpm exec nx run-many -t lint test build --projects=portal-shell` — green (19 / 19 specs).
- [x] Production build under bundle budgets (100 kB gzip initial).
- [ ] Manual: load `/`, confirm Dashboard appears active in the sidebar, collapse → reload → still collapsed, focus the address bar then Tab → skip-link visible, keyboard-traverse the sidebar.
- [ ] Manual: `prefers-reduced-motion: reduce` → no width animation when toggling.
- [ ] Manual: zoom to 200 % → no horizontal scroll, header search hides at narrow widths (`md:` breakpoint).

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #83

apps/portal-shell/src/styles.css

@@ -8,3 +8,46 @@
  */
 
 @import 'tailwindcss';
+
+/*
+ * Brand palette — APF charte graphique.
+ *
+ *   primary: #12546c (dark teal)  — header bar, sidebar active items,
+ *                                   primary buttons, headings on light bg
+ *   accent:  #f7a919 (warm orange) — CTAs, highlights, focus rings on
+ *                                    branded surfaces
+ *
+ * The two scales below are hand-tuned (each step roughly halves the
+ * delta from the previous toward white / black). Tailwind v4 surfaces
+ * them automatically as utility classes — `bg-brand-primary-700`,
+ * `text-brand-accent-500`, `ring-brand-primary-500`, etc.
+ *
+ * Use the `500` step as the canonical brand value; the lighter / darker
+ * steps are for hover / focus / muted text without dropping out of the
+ * brand family.
+ */
+@theme {
+  --color-brand-primary-50: #eaf3f7;
+  --color-brand-primary-100: #cae0e8;
+  --color-brand-primary-200: #9ac4d2;
+  --color-brand-primary-300: #5fa2b6;
+  --color-brand-primary-400: #2f7f96;
+  --color-brand-primary-500: #12546c;
+  --color-brand-primary-600: #0f475c;
+  --color-brand-primary-700: #0c394a;
+  --color-brand-primary-800: #082a36;
+  --color-brand-primary-900: #051c24;
+  --color-brand-primary-950: #020e12;
+
+  --color-brand-accent-50: #fef5e0;
+  --color-brand-accent-100: #fce6b3;
+  --color-brand-accent-200: #fad17a;
+  --color-brand-accent-300: #f8bb47;
+  --color-brand-accent-400: #f7a919;
+  --color-brand-accent-500: #de9415;
+  --color-brand-accent-600: #b97a11;
+  --color-brand-accent-700: #93600d;
+  --color-brand-accent-800: #6e4709;
+  --color-brand-accent-900: #4a3006;
+  --color-brand-accent-950: #271903;
+}