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/not-found.tsx

@@ -0,0 +1,24 @@
+import Link from "next/link";
+import { AppShell } from "@/components/app-shell";
+import { Button } from "@/components/ui/button";
+
+export default function NotFound() {
+  return (
+    <AppShell>
+      <section className="max-w-xl mx-auto text-center py-16 space-y-5">
+        <div className="font-display text-gold text-6xl leading-none" aria-hidden="true">
+          404
+        </div>
+        <div className="space-y-2">
+          <h1 className="text-navy">הדף לא נמצא</h1>
+          <p className="text-ink-muted leading-relaxed">
+            הכתובת שביקשת אינה קיימת או שהוזזה לדף אחר.
+          </p>
+        </div>
+        <Button asChild className="bg-navy hover:bg-navy-soft text-parchment">
+          <Link href="/">חזרה לבית</Link>
+        </Button>
+      </section>
+    </AppShell>
+  );
+}