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

@@ -182,6 +182,10 @@ function handleTextareaMouseUp() {
 			onmouseup={handleTextareaMouseUp}
 		></textarea>
 
+		{#if selectedQuote}
+			<p class="mt-1 text-xs text-ink-3">{m.transcription_block_quote_hint()}</p>
+		{/if}
+
 		<!-- Footer -->
 		<div class="flex items-center justify-between border-t border-line pt-2">
 			<div>