feat(captions): edit image/video captions after sending

Captions could be attached at upload but never changed — canEditEvent
only allowed m.text/emote/notice, so a typo in an image caption meant
delete + re-upload. Add caption editing for image/video messages.

- utils/room.ts: canEditCaption (own image/video RoomMessage, no
  non-thread relation) + canEditEventOrCaption. canEditEvent unchanged.
- Message.tsx: gate the Edit affordance (quick-actions + menu) on
  canEditEventOrCaption; label it "Edit caption" for media; keep the media
  rendered above the editor while editing.
- MessageEditor.tsx: for a media message, seed the editor from the caption
  (not the filename), allow an empty caption (removes it), and build the
  m.replace so m.new_content spreads the original media content
  (url/info/encrypted file/filename/msgtype) and only sets body +
  format/formatted_body. Outer content is the full media (not a "* text"
  fallback) so non-edit-aware clients still render the media. No-op guard
  when the caption is unchanged. Placeholder "Add a caption…".

Rendering + Edit History need no changes: getEditedEvent's m.new_content
flows to renderCaption, and the word-diff already diffs body (the
caption). Encrypted media keeps its file/key (no re-upload).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/message/Message.tsx

@@ -50,7 +50,8 @@ import {
   UsernameBold,
 } from '../../../components/message';
 import {
-  canEditEvent,
+  canEditCaption,
+  canEditEventOrCaption,
   getEventEdits,
   getMemberAvatarMxc,
   getMemberName,
@@ -903,17 +904,22 @@ export const Message = React.memo(
         <Box direction="Column" alignSelf="Start" style={{ maxWidth: '100%' }}>
           {reply}
           {edit && onEditId ? (
-            <MessageEditor
-              style={{
-                maxWidth: '100%',
-                width: '100vw',
-              }}
-              roomId={room.roomId}
-              room={room}
-              mEvent={mEvent}
-              imagePackRooms={imagePackRooms}
-              onCancel={() => onEditId()}
-            />
+            <>
+              {/* Editing a media caption: keep the media visible for context; the
+                  editor below edits only the caption. */}
+              {canEditCaption(mx, mEvent) && children}
+              <MessageEditor
+                style={{
+                  maxWidth: '100%',
+                  width: '100vw',
+                }}
+                roomId={room.roomId}
+                room={room}
+                mEvent={mEvent}
+                imagePackRooms={imagePackRooms}
+                onCancel={() => onEditId()}
+              />
+            </>
           ) : (
             children
           )}
@@ -1072,13 +1078,13 @@ export const Message = React.memo(
                       <Icon src={Icons.ThreadPlus} size="100" />
                     </IconButton>
                   )}
-                  {canEditEvent(mx, mEvent) && onEditId && (
+                  {canEditEventOrCaption(mx, mEvent) && onEditId && (
                     <IconButton
                       onClick={() => onEditId(mEvent.getId())}
                       variant="SurfaceVariant"
                       size="300"
                       radii="300"
-                      aria-label="Edit message"
+                      aria-label={canEditCaption(mx, mEvent) ? 'Edit caption' : 'Edit message'}
                     >
                       <Icon src={Icons.Pencil} size="100" />
                     </IconButton>
@@ -1247,7 +1253,7 @@ export const Message = React.memo(
                                 </Text>
                               </MenuItem>
                             )}
-                            {canEditEvent(mx, mEvent) && onEditId && (
+                            {canEditEventOrCaption(mx, mEvent) && onEditId && (
                               <MenuItem
                                 size="300"
                                 after={<Icon size="100" src={Icons.Pencil} />}
@@ -1264,7 +1270,7 @@ export const Message = React.memo(
                                   size="T300"
                                   truncate
                                 >
-                                  Edit Message
+                                  {canEditCaption(mx, mEvent) ? 'Edit Caption' : 'Edit Message'}
                                 </Text>
                               </MenuItem>
                             )}