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/lib/components/TranscriptionBlock.svelte.spec.ts

@@ -228,12 +228,12 @@ describe('TranscriptionBlock — delete confirmation', () => {
 describe('TranscriptionBlock — quote selection', () => {
 	it('shows quote hint after text is selected in textarea', async () => {
 		renderBlock({ text: 'Breslau, den 12. August' });
-		const textarea = page.getByRole('textbox');
-		// Select all text via keyboard shortcut to trigger mouseup with selection
-		await textarea.click();
-		await textarea.selectText();
-		// Fire mouseup to trigger the selection handler
-		await textarea.dispatchEvent('mouseup');
+		await page.getByRole('textbox').click();
+		// Select text and fire mouseup via native DOM — locator.selectText/dispatchEvent not available
+		const el = document.querySelector('textarea') as HTMLTextAreaElement;
+		el.focus();
+		el.setSelectionRange(0, el.value.length);
+		el.dispatchEvent(new MouseEvent('mouseup', { bubbles: true }));
 		await expect.element(page.getByText(/Zitat/)).toBeInTheDocument();
 	});
 });