feat(chat): edit message (#4)

Sender can edit their own text messages: a pencil action on the bubble loads the text into the
composer in an 'Editing' mode; saving updates the body, marks it 'edited', and pushes the change
live to the other side/tabs (chat-edited, mirroring delete). Adds messages.edited_at + editBody().

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

server/repos.js

@@ -188,6 +188,7 @@ const messages = {
   byAttachment: (attachmentId) => db.prepare('SELECT * FROM messages WHERE attachment_id=? LIMIT 1').get(attachmentId),
   setPoll: (messageId, pollId) => db.prepare('UPDATE messages SET poll_id=? WHERE id=?').run(pollId, messageId),
   markDelivered: (id) => db.prepare('UPDATE messages SET delivered_at=? WHERE id=? AND delivered_at IS NULL').run(now(), id),
+  editBody: (id, body) => db.prepare('UPDATE messages SET body=?, edited_at=? WHERE id=?').run(body, now(), id),
   // Delete-for-everyone: clear the content but keep the row (renders as a placeholder).
   markDeleted: (id) => db.prepare("UPDATE messages SET deleted=1, body='', attachment_id=NULL, poll_id=NULL WHERE id=?").run(id),
   // Shared media/files in a conversation (group) or DM — newest first.