Add quote indicator and distinct callout styling for Note/Tip/Warning/Important

- Description editor: blockquotes get a left accent line and muted color so
  it's clear when typing in "quote mode" (matches the existing active Quote
  toolbar button highlight).
- Editor block cards: each text block's left border is colored by its level
  (info/success/warn/error) so Note/Tip/Warning/Important are distinguishable
  at a glance.
- PDF/HTML/DOCX exports: callouts now get a level-specific accent color and
  tinted background/shading (blue/green/amber/red), instead of all looking
  identical except for the label text.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

app/renderer/editor.js

@@ -674,6 +674,7 @@ class GuideEditor {
       }, header);
 
       if (kind === 'text') {
+        card.dataset.level = block.level || 'info';
         const position = makeSelect(block.position, [
           { value: 'before-title', label: 'Before title' },
           { value: 'after-title', label: 'After title' },
@@ -696,7 +697,7 @@ class GuideEditor {
         body.dataset.blockField = 'body';
         body.value = (block.descriptionHtml || '').replace(/<[^>]+>/g, '');
         position.addEventListener('change', () => { block.position = position.value; save(); });
-        level.addEventListener('change', () => { block.level = level.value; save(); });
+        level.addEventListener('change', () => { block.level = level.value; card.dataset.level = level.value; save(); });
         title.addEventListener('input', () => { block.title = title.value; save(); });
         body.addEventListener('input', () => { block.descriptionHtml = `<p>${escapeHtml(body.value)}</p>`; save(); });
         card.append(