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/package.json

@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.52.2",
+  "version": "0.52.3",
   "type": "module",
   "scripts": {
     "dev": "vite",