fix(routing): render a 404 page for unmatched routes (v0.52.3)

The top-level catch-all silently redirected unknown paths to "/", and the
nested /admin/*, /p/:projectId/*, and /docs/* route groups had no catch-all
(invalid subpaths rendered a blank pane). Add a shared NotFound component and
wire it into all four route groups so a bad/typo'd URL shows a clear 404 with
a link home. Client-side 404 UI (SPA still serves HTTP 200).

Patch 0.52.2 → 0.52.3; CHANGELOG updated. Caught by the operator on PPE.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/App.jsx

@@ -7,6 +7,7 @@ import { brandTitle } from './lib/brand'
 import { entryPath, proposalPath, DEFAULT_COLLECTION } from './lib/entryPaths'
 import { useDeployment } from './context/DeploymentProvider'
 import ProjectLayout from './components/ProjectLayout.jsx'
+import NotFound from './components/NotFound.jsx'
 import Directory from './components/Directory.jsx'
 import ProjectSwitcher from './components/ProjectSwitcher.jsx'
 import Catalog from './components/Catalog.jsx'
@@ -381,13 +382,14 @@ export default function App() {
                   <Route path="c/:collectionId/e/:slug" element={<RFCView viewer={viewer} />} />
                   <Route path="c/:collectionId/e/:slug/pr/:prNumber" element={<PRView viewer={viewer} />} />
                   <Route path="c/:collectionId/proposals/:prNumber" element={<ProposalView viewer={viewer} onChange={() => setCatalogVersion(v => v + 1)} />} />
+                  <Route path="*" element={<NotFound />} />
                 </Routes>
               </main>
             </ProjectLayout>
           } />
-          {/* Any other path (incl. the retired bare-slug corpus URLs that
-              somehow reach the SPA) lands on the deployment landing. */}
-          <Route path="*" element={<Navigate to="/" replace />} />
+          {/* Any unmatched path is a real 404 — surface it rather than
+              silently bouncing home, so a bad/typo'd URL is visible. */}
+          <Route path="*" element={<NotFound />} />
         </Routes>
       </div>
       {(proposeOpen || proposeParam != null) && viewer && (
@@ -507,6 +509,7 @@ function DocsWithSidebar({ viewer }) {
               client-side redirect to the first configured spec. */}
           <Route path="specs" element={<DocsSpecsIndex />} />
           <Route path="specs/:name" element={<DocsSpec />} />
+          <Route path="*" element={<NotFound />} />
         </Route>
       </Routes>
     </main>