chore(editor): address image-caption review (#221)

- docs: add CHANGELOG Unreleased/Added entry for editable image captions
- test: export sanitizeCaption and add vitest unit coverage
  (whitespace collapse, trim, 500-char boundary)
- refactor: drop duplicate .imageCaption CSS module class, keep the
  global .image-caption as the single source
- docs: fix turndown image-caption comment (video rule emits a markdown
  link, not a <div>)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/editor/components/image/image-view.module.css

@@ -7,16 +7,6 @@
   overflow: hidden;
 }
 
-.imageCaption {
-  display: block;
-  text-align: center;
-  font-size: 0.875em;
-  color: var(--mantine-color-dimmed);
-  margin-top: 0.4em;
-  line-height: 1.35;
-  word-break: break-word;
-}
-
 .skeleton {
   animation: pulse 1.2s ease-in-out infinite;
 

apps/client/src/features/editor/components/image/image-view.tsx

@@ -72,7 +72,7 @@ export default function ImageView(props: NodeViewProps) {
       {captionText && (
         <Text
           component="figcaption"
-          className={clsx(classes.imageCaption, "image-caption")}
+          className="image-caption"
         >
           {captionText}
         </Text>