fix: correct route registration - use full paths in rjweb-server path.http() calls

rjweb-server path.http() second argument is the FULL path, not relative to prefix.
Changed all routes from server.path('/prefix', path => path.http(method, '/suffix', ...))
to server.path('/', path => path.http(method, '/prefix/suffix', ...)).

Also fixed:
- Duplicate createContext import in useAuth.ts
- notFound handler now excludes /webhook paths from SPA fallback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/hooks/useAuth.ts

@@ -21,8 +21,7 @@ interface AuthContextType {
   refresh: () => Promise<void>;
 }
 
-import { createContext as _createContext } from "react";
-export const AuthContext = _createContext<AuthContextType>({
+export const AuthContext = createContext<AuthContextType>({
   user: null,
   loading: true,
   refresh: async () => {},