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/CommentThread.svelte

@@ -199,7 +199,9 @@ onMount(() => {
 });
 </script>
 
-{#if flatMessages.length > 0}
+{#if flatMessages.length === 0}
+	<p class="text-sm text-ink-3 italic">{m.comment_empty_hint()}</p>
+{:else}
 	<div class="rounded border-l-2 border-accent bg-muted p-2">
 		<div class="mb-2 flex items-center gap-1.5 font-sans text-sm font-semibold text-ink-2">
 			<svg