test(stammbaum): fix two CI-only browser-test failures (#692)

- page.svelte.test.ts mocked $app/navigation with only replaceState, dropping
  invalidateAll (imported by StammbaumSidePanel) → the module errored and failed
  all 7 tests in the file. Mock now exports invalidateAll + goto too.
- StammbaumTree viewBox 'offsets origin' test hard-coded a wrong unpanned-x; assert
  the robust relationship instead (viewBox centre − content centroid == pan).

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

frontend/src/routes/stammbaum/page.svelte.test.ts

@@ -17,7 +17,11 @@ vi.mock('$app/state', () => ({
 
 const replaceState = vi.fn();
 vi.mock('$app/navigation', () => ({
-	replaceState: (...args: unknown[]) => replaceState(...args)
+	replaceState: (...args: unknown[]) => replaceState(...args),
+	// StammbaumSidePanel (rendered transitively) imports invalidateAll/goto, so
+	// the mock must provide every export the module graph uses.
+	invalidateAll: vi.fn(),
+	goto: vi.fn()
 }));
 
 afterEach(cleanup);