fix(tests): resolve all 4 pre-existing test failures

- CommentThread: add missing empty-state paragraph using comment_empty_hint
  i18n key (key existed but was never rendered in the template)
- TranscriptionBlock: add selectedQuote hint using transcription_block_quote_hint
  i18n key (key existed but was never rendered); fix test to use native DOM
  el.focus()/setSelectionRange()/dispatchEvent instead of locator.selectText()
  which is not available in this vitest-browser version
- TranscriptionEditView: fix test to use native el.dispatchEvent(FocusEvent)
  instead of locator.blur() which is not available
- Conversations: fix test expected text from stale "Korrespondenz durchsuchen"
  to match current conv_empty_heading() = "Wessen Briefe möchten Sie lesen?"

All 687 tests now pass.

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

frontend/src/routes/conversations/page.svelte.spec.ts

@@ -50,7 +50,7 @@ const withDocs = {
 describe('Conversations page – empty state', () => {
 	it('shows the empty-state heading when no persons are selected', async () => {
 		render(Page, { data: baseData });
-		await expect.element(page.getByText(/Korrespondenz durchsuchen/i)).toBeInTheDocument();
+		await expect.element(page.getByText(/Wessen Briefe möchten Sie lesen/i)).toBeInTheDocument();
 	});
 
 	it('hides the swap button when no persons are selected', async () => {