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/components/training/corpus-panel.tsx

@@ -79,9 +79,10 @@ function Row({ item }: { item: CorpusDecision }) {
           size="sm"
           onClick={onDelete}
           disabled={del.isPending}
+          aria-label={`הסר את ${item.decision_number || "החלטה זו"} מהקורפוס`}
           className="text-danger hover:text-danger hover:bg-danger-bg"
         >
-          <Trash2 className="w-4 h-4" />
+          <Trash2 className="w-4 h-4" aria-hidden="true" />
         </Button>
       </TableCell>
     </TableRow>