refactor(libs): graduate Icon / LayoutStateService / brand tokens to libs/shared/* (#99)

## Summary

Per ADR-0020 §"Shared-libs graduation": the three primitives that `portal-shell` (today) and `portal-admin` (next) will both share move out of `apps/portal-shell/src/` and into the workspace libs. Mechanical move — no behaviour change, prepares the ground for the admin-app PR.

## Graduated

| Primitive | Old location | New location |
|---|---|---|
| `Icon` component (+ `IconName` type) | `apps/portal-shell/src/app/components/icon/` | [`libs/shared/ui/src/lib/icon/`](libs/shared/ui/src/lib/icon/) |
| `LayoutStateService` (+ `ThemeMode` type) | `apps/portal-shell/src/app/state/` | [`libs/shared/state/src/lib/`](libs/shared/state/src/lib/) |
| Brand-palette `@theme` block | `apps/portal-shell/src/styles.css` | [`libs/shared/tokens/src/brand-tokens.css`](libs/shared/tokens/src/brand-tokens.css) |

## Notable changes

- **Icon selector renamed `app-icon` → `lib-icon`.** Required by the lib's ESLint `@angular-eslint/component-selector` rule (prefix `lib` for shared libs). All ~20 usages in `portal-shell` templates updated in one sweep.
- **New `shared-state` library** generated via `nx g @nx/angular:library libs/shared/state`. Mirrors the existing `feature-auth` / `shared-ui` shape (vite + Angular + spec setup). `tsconfig.base.json` path alias `shared-state` added by the generator.
- **Lib tags rebalanced** to satisfy the module-boundary lint rule:
  - `shared-state`: new lib → `scope:shared, type:shared`.
  - `shared-ui`: was `scope:portal-shell` (scaffold default) → `scope:shared, type:shared`.
- **`shared-tokens` is now CSS-only.** The placeholder TS function is gone; `index.ts` is an empty `export {}` with a TODO note. Vitest config flips `passWithNoTests: true` so the test target stops failing on the empty lib.
- **`portal-shell`'s `styles.css`** drops the inline `@theme {}` block and instead `@import`s `../../../libs/shared/tokens/src/brand-tokens.css`. Both apps will read the same source.
- Placeholder scaffolded files in `shared-ui/src/lib/shared-ui/` removed.

## Verification

- `nx run-many -t lint test build` across `portal-shell, shared-ui, shared-state, shared-tokens` — green.
- Tests redistributed: **portal-shell 28**, **shared-state 9**, **shared-ui 3**, **shared-tokens 0** = 40 total (same as before the move).
- Production build: **128.7 KB gzip** initial per locale (was 128.5 KB on `main` — noise-level delta).
- Brand-color CSS variables (`--color-brand-primary-500: #12546c`, …) still inlined in the produced `styles-*.css`.
- `dist/.../browser/{en,fr}/` emitted as expected.

## What this PR explicitly does NOT do

- Move other components (Header, Sidebar, Footer, ThemeSwitcher, LocaleSwitcher). Those are app-specific shell concerns; if `portal-admin` ends up needing them, they graduate in their own PR.
- Set up `portal-admin`. That's the next PR on the admin track — and now imports from `shared-ui` / `shared-state` / `shared-tokens` will Just Work.
- Refactor `feature-auth`'s tagging. It still says `scope:portal-shell`; revisit when the auth implementation actually lands and the dual-audience design (per ADR-0008) makes the right scope obvious.

## Test plan

- [x] Per-project run-many — green.
- [x] Production build emits both locales with brand tokens applied.
- [ ] Manual: `pnpm exec nx serve portal-shell` — the dev experience is unchanged.
- [ ] Manual: serve-static + locale switcher / theme switcher / sidebar collapse / navigation — all the behaviours the moved primitives drive still work in production.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #99

libs/shared/state/tsconfig.lib.json

@@ -0,0 +1,26 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "outDir": "../../../dist/out-tsc",
+    "declaration": true,
+    "declarationMap": true,
+    "inlineSources": true,
+    "types": []
+  },
+  "include": ["src/**/*.ts"],
+  "exclude": [
+    "src/**/*.spec.ts",
+    "src/**/*.test.ts",
+    "vite.config.ts",
+    "vite.config.mts",
+    "vitest.config.ts",
+    "vitest.config.mts",
+    "src/**/*.test.tsx",
+    "src/**/*.spec.tsx",
+    "src/**/*.test.js",
+    "src/**/*.spec.js",
+    "src/**/*.test.jsx",
+    "src/**/*.spec.jsx",
+    "src/test-setup.ts"
+  ]
+}