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/TranscriptionEditView.svelte.spec.ts

@@ -193,8 +193,9 @@ describe('TranscriptionEditView — flush on blur', () => {
 		const textarea = page.getByRole('textbox').first();
 		await textarea.fill('Blur text');
 
-		// Blur before 1500ms debounce fires
-		await textarea.blur();
+		// Blur before 1500ms debounce fires — locator.blur() not available, use native DOM
+		const el = document.querySelector('textarea') as HTMLTextAreaElement;
+		el.dispatchEvent(new FocusEvent('blur', { bubbles: true }));
 
 		await vi.runAllTimersAsync();
 		expect(onSaveBlock).toHaveBeenCalledWith('b1', 'Blur text');