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/db.js

@@ -216,6 +216,8 @@ try { db.exec('ALTER TABLE messages ADD COLUMN msg_type TEXT'); } catch (e) { /*
 // Deleted ("delete for everyone"): the row stays so threads/ordering hold, but body+attachment
 // are cleared and clients render a "This message was deleted" placeholder.
 try { db.exec('ALTER TABLE messages ADD COLUMN deleted INTEGER NOT NULL DEFAULT 0'); } catch (e) { /* exists */ }
+// A message can be edited by its sender; edited_at marks it (shows an "edited" label).
+try { db.exec('ALTER TABLE messages ADD COLUMN edited_at INTEGER'); } catch (e) { /* exists */ }
 // User-set presence status: 'active' | 'away' | 'onleave'. ('incall' is derived live, not stored.)
 try { db.exec("ALTER TABLE users ADD COLUMN status TEXT NOT NULL DEFAULT 'active'"); } catch (e) { /* exists */ }
 // BizGaze person-id (s.userId): the SAME value whether the person signs in with their email or