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/app/app.scss

@@ -1,10 +1,26 @@
-// Page-level layout: header + main + footer in a flex column so the
-// footer sticks to the viewport bottom even when the main content is
-// short. Driven on the host so it covers the whole document.
+// App shell: fixed-height header on top, sidebar + main beside each
+// other below, both filling the remaining viewport height. The sidebar
+// owns its own scrolling (long menus don't push the header off-screen);
+// the main pane scrolls independently so opening a long page never
+// scrolls the sidebar out of view.
 :host {
   display: flex;
   flex-direction: column;
   min-height: 100vh;
+  height: 100vh;
+}
+
+.shell-body {
+  display: flex;
+  flex: 1 1 auto;
+  min-height: 0;
+}
+
+.shell-main {
+  flex: 1 1 auto;
+  min-width: 0;
+  overflow-y: auto;
+  background-color: #f9fafb;
 }
 
 // Skip-link (WCAG 2.4.1 "Bypass Blocks"). Hidden by default, fully
@@ -17,7 +33,7 @@
   left: 0.75rem;
   z-index: 50;
   padding: 0.5rem 0.75rem;
-  background: #1d4ed8;
+  background: var(--color-brand-primary-500);
   color: #fff;
   border-radius: 0.25rem;
   text-decoration: none;
@@ -26,7 +42,7 @@
   &:focus,
   &:focus-visible {
     top: 0.5rem;
-    outline: 2px solid #93c5fd;
+    outline: 2px solid var(--color-brand-accent-300);
     outline-offset: 2px;
   }
 }