Phase 6: polish — error boundaries, a11y, smoke test doc

Close out the read-only surface before cutover with three families of
small fixes that the previous phases left unfinished:

- Error boundaries: add src/app/error.tsx (route-segment), global-error.tsx
  (root crash fallback with its own minimal html/body — no Providers
  dependency since those may be the thing that crashed), and not-found.tsx
  for a Hebrew 404 instead of the default Next page.

- Accessibility: wire usePathname() into AppShell so the current nav item
  gets aria-current="page" and a gold underline. Add aria-label + aria-hidden
  on the icon-only buttons that Phase 5 left text-less (corpus trash,
  parties-field Plus). Nav gets an aria-label of its own.

- Metadata template: title on each route now reads "X · עוזר משפטי" via
  the layout.tsx title.template. Description localized to Jerusalem.

- README: full E2E smoke test checklist covering all 9 screens, plus a
  backend contract table so future phases know which hook wraps which
  endpoint. Documents the known Gitea→Coolify webhook issue.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web-ui/src/app/layout.tsx

@@ -14,8 +14,11 @@ const heebo = Heebo({
 });
 
 export const metadata: Metadata = {
-  title: "עוזר משפטי — ניהול תיקים",
-  description: "מערכת סיוע בניסוח החלטות לוועדת ערר לתכנון ובנייה",
+  title: {
+    default: "עוזר משפטי — ניהול תיקים",
+    template: "%s · עוזר משפטי",
+  },
+  description: "מערכת סיוע בניסוח החלטות לוועדת ערר לתכנון ובנייה, ירושלים",
 };
 
 export default function RootLayout({