feat(web): deep-linkable field selection via /fields/:key (#72)

FieldsPage kept the selected field definition in component state, so
reload lost the selection, fields couldn't be linked/shared, and
back/forward didn't navigate selections — inconsistent with
/vocabularies/:id and /objects/:id.

Move selection into the URL: the route becomes /fields/:key?
(optional segment), FieldList selection navigates, cancel/done
navigates back to /fields, and the page derives the selected def from
the already-cached field-defs query. An unknown or stale key (e.g.
after deleting the selected field) falls back to the create form.

Tests: deep link opens the locked edit form, select→cancel round-trips
through the URL, unknown key falls back to create.

Closes #72

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/app.tsx

@@ -72,7 +72,7 @@ const router = createBrowserRouter(
           <Route path="/authorities" element={<Navigate to="/authorities/person" replace />} />
           <Route path="/authorities/:kind" element={<AuthoritiesPage />} />
           <Route
-            path="/fields"
+            path="/fields/:key?"
             element={
               <Suspense fallback={<ListSkeleton />}>
                 <FieldsPage />