Sponsor Chat: two-way member<->sponsor messaging (presence, availability, mute)

Evolves the one-way broadcast into real support threads, keeping broadcast alongside.
- messages.js: kind='chat' rides sponsor_messages; sendChat/thread/threadList/
  markChatRead/chatUnread; broadcast inbox + login modal scoped to kind='broadcast'
- accounts.js: presence (last_seen), chat availability toggle, per-member mutes;
  sponsorOf() + isDownlineOf() resolvers
- server.js: /api/my/chat/{send,thread,threads,available,mute} + /api/my/ping
  heartbeat; dashboard emits chatUnread/chatAvailable/sponsor; auth = direct
  sponsor up, any downline down, or an existing thread; email only when offline
  and not mid-chat
- my.html/my.js/site.css: slide-in chat drawer (threads list + live thread,
  4s poll), presence dot, Message buttons on direct rows, Message-my-sponsor
  quick action, availability switch in Profile
- db.js: additive migrations (kind, pair index, chat_available, chat_mutes)
- chatbot.js: canned answer + AI fact for Sponsor Chat

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

db.js

@@ -136,6 +136,12 @@ async function bootstrap() {
     read_ts BIGINT NULL,
     INDEX (to_email, read_ts), INDEX (from_email, sent)
   ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`);
+  // sponsor CHAT (two-way) rides the same table: kind='chat' vs the default 'broadcast'
+  await alterSafe(`ALTER TABLE sponsor_messages ADD COLUMN kind VARCHAR(12) NOT NULL DEFAULT 'broadcast'`);
+  await alterSafe('ALTER TABLE sponsor_messages ADD INDEX idx_pair (to_email, from_email)');
+  // per-account chat settings: availability toggle (default on) + muted-member list (JSON emails)
+  await alterSafe('ALTER TABLE accounts ADD COLUMN chat_available TINYINT NOT NULL DEFAULT 1');
+  await alterSafe('ALTER TABLE accounts ADD COLUMN chat_mutes VARCHAR(4000) NULL');
   await q(`CREATE TABLE IF NOT EXISTS burns (
     id VARCHAR(32) PRIMARY KEY,
     member_id INT NOT NULL,