refactor: split dashboard.page.ts into per-concern sections

Each dashboard section (Mijn aanvragen, Wat moet ik regelen, Mijn
registratie, Specialismen, Wat wilt u doen, Beheer) now owns its own
store access, async state, and template. DashboardPage becomes pure
composition.

Extract the repeated RemoteData Success-narrowing pattern into
successOf() and the dashboard sort/split logic into
sortForDashboard/concepten/ingediend, both with tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

libs/shared/src/testing/remote-data.ts

@@ -11,3 +11,5 @@ export const loading = <E = never, T = never>(): RemoteData<E, T> => ({ tag: 'Lo
 export const success = <T, E = never>(value: T): RemoteData<E, T> => ({ tag: 'Success', value });
 
 export const failure = <E, T = never>(error: E): RemoteData<E, T> => ({ tag: 'Failure', error });
+
+export const empty = <E = never, T = never>(): RemoteData<E, T> => ({ tag: 'Empty' });