#10 deleted-last-message preview + #18 delete for me / for everyone

#10: deleting the last message showed "No messages yet" in the chat list.
The sidebar sent an empty last_body for a deleted (content-cleared) row; now
it sends a last_deleted flag and the row renders "This message was deleted"
(or "You deleted this message"), matching the in-thread placeholder.

#18: added "Delete for me" alongside "Delete for everyone". A new message_hidden
table records a per-user hide; the thread + sidebar (last message, unread) filter
out the requesting user's hidden messages, and the hide is echoed to their other
devices (chat-hidden). "Delete for me" is offered on any message; "Delete for
everyone" stays sender-only.

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

server/db.js

@@ -216,6 +216,9 @@ 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 */ }
+// #18 "Delete for me": a per-user hide. The message row is untouched (others still see it); this
+// table records that THIS user removed it from their own view (threads + sidebar filter it out).
+try { db.exec('CREATE TABLE IF NOT EXISTS message_hidden (message_id TEXT NOT NULL, user_id TEXT NOT NULL, hidden_at INTEGER, PRIMARY KEY (message_id, user_id))'); } 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 */ }
 try { db.exec('ALTER TABLE messages ADD COLUMN fwd_from TEXT'); } catch (e) { /* exists — original sender name when a message was forwarded (#5) */ }