feat(app): route every page with wouter

Replace the single-scroll showcase and its anchor/scrollIntoView
navigation with real routes: /, /patterns, /components/*, /foundations/*,
/docs/*, plus a 404 fallback.

- src/app/routes.tsx is the route table and the only source for paths,
  section headings, sidebar entries, breadcrumbs and the rail, so
  navigation cannot drift from the pages that exist
- src/pages/* holds one component per route; App.tsx is now the shell
  (topbar, sidebar, breadcrumbs, Switch, right rail, scroll-to-top)
- the active pattern moves into the URL (/patterns?pattern=table) and the
  sidebar/palette deep-link into it; SearchPalette results now carry a path
- Sidebar/RightRail render <Link> with aria-current; Hero and Topbar use
  route links too (ButtonLink keeps the button visual)
- PATTERNS + PatternId move to src/data/patterns.ts (no non-component
  export in a component file), pattern buttons gain aria-pressed
- tests/routing.test.tsx: registry/nav invariants, per-route rendering,
  sidebar navigation, pattern deep links, 404

README.md

@@ -46,7 +46,8 @@ src/
   index.css         ← fonts, token import, base resets
   utils.css         ← token-compliant utility classes
   App.css           ← application showcase styles
-  app/              ← AppStateProvider + useAppState(): theme, viewMode, sidebar, ⌘K search
+  app/              ← AppStateProvider + useAppState(), and the route table (wouter)
+  pages/            ← one page component per route
   components/
     ui/             ← composable primitives (Overlay, Dialog, Sheet, Tabs, Alert, …)
     icon-registry.ts← the one icon name → lucide component map