feat(editor): add editable image captions (#221)

Add a visible caption (<figcaption>) under images, editable from the
image bubble-menu and persisted across all formats: native Yjs/JSON,
HTML export, and Markdown.

- image node: new plain-text `caption` attribute (parse/render
  `data-caption` on <img>, emitted only when set) + `setImageCaption`
  command. The node stays an atom; the schema shape is unchanged, so the
  server's generateHTML/generateJSON path round-trips it for free.
- resize node-view: re-parent the resizable wrapper into a <figure> and
  render the caption in a <figcaption> BELOW it, outside nodeView.wrapper
  (so onCommit's offsetHeight measurement and the left/right resize
  handles still cover the image only). This path also drives read-only /
  share rendering. React placeholder view renders the caption too.
- bubble-menu: new useCaptionControl panel modeled on useAltTextControl
  (own icon, Caption strings, softer sanitizer, ~500 char limit).
- markdown lossless round-trip: a captioned image is emitted as a raw
  <img data-caption> wrapped in a block <div> (same trick as <video>) in
  both the editor-ext turndown rule and the MCP converter; caption-less
  images stay clean ![alt](src). Import restores the caption via the
  shared markdownToHtml + parseHTML.
- styles + i18n keys; tests for the schema attr round-trip, markdown
  round-trip (editor-ext) and the MCP converter.

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

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

@@ -23,6 +23,7 @@ import { useTranslation } from "react-i18next";
 import { getFileUrl } from "@/lib/config.ts";
 import { uploadImageAction } from "@/features/editor/components/image/upload-image-action.tsx";
 import { useAltTextControl } from "@/features/editor/components/common/use-alt-text-control.tsx";
+import { useCaptionControl } from "@/features/editor/components/common/use-caption-control.tsx";
 import classes from "../common/toolbar-menu.module.css";
 
 export function ImageMenu({ editor }: EditorMenuProps) {
@@ -47,6 +48,7 @@ export function ImageMenu({ editor }: EditorMenuProps) {
         isFloatRight: ctx.editor.isActive("image", { align: "floatRight" }),
         src: imageAttrs?.src || null,
         alt: imageAttrs?.alt || "",
+        caption: imageAttrs?.caption || "",
       };
     },
   });
@@ -168,6 +170,16 @@ export function ImageMenu({ editor }: EditorMenuProps) {
     currentAlt: editorState?.alt || "",
   });
 
+  const {
+    button: captionButton,
+    panel: captionPanel,
+    isEditing: isEditingCaption,
+  } = useCaptionControl({
+    editor,
+    nodeName: "image",
+    currentCaption: editorState?.caption || "",
+  });
+
   return (
     <BaseBubbleMenu
       editor={editor}
@@ -183,6 +195,8 @@ export function ImageMenu({ editor }: EditorMenuProps) {
     >
       {isEditingAlt ? (
         altTextPanel
+      ) : isEditingCaption ? (
+        captionPanel
       ) : (
         <div className={classes.toolbar}>
         <Tooltip position="top" label={t("Align left")} withinPortal={false}>
@@ -249,6 +263,8 @@ export function ImageMenu({ editor }: EditorMenuProps) {
 
         {altTextButton}
 
+        {captionButton}
+
         <div className={classes.divider} />
 
         <Tooltip position="top" label={t("Download")} withinPortal={false}>

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

@@ -7,6 +7,16 @@
   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

@@ -9,7 +9,9 @@ import { useTranslation } from "react-i18next";
 export default function ImageView(props: NodeViewProps) {
   const { t } = useTranslation();
   const { editor, node, selected } = props;
-  const { src, width, align, alt, aspectRatio, placeholder } = node.attrs;
+  const { src, width, align, alt, caption, aspectRatio, placeholder } =
+    node.attrs;
+  const captionText = (caption || "").trim();
   const alignClass = useMemo(() => {
     if (align === "left") return "alignLeft";
     if (align === "right") return "alignRight";
@@ -29,6 +31,7 @@ export default function ImageView(props: NodeViewProps) {
 
   return (
     <NodeViewWrapper data-drag-handle>
+      <figure style={{ margin: 0 }}>
       <div
         className={clsx(
           selected && "ProseMirror-selectednode",
@@ -66,6 +69,15 @@ export default function ImageView(props: NodeViewProps) {
           </Group>
         )}
       </div>
+      {captionText && (
+        <Text
+          component="figcaption"
+          className={clsx(classes.imageCaption, "image-caption")}
+        >
+          {captionText}
+        </Text>
+      )}
+      </figure>
     </NodeViewWrapper>
   );
 }