feat(social, multiplayer): working chat and groups, reliable challenges, true VS, resume sync, emotes from the sender

Social: chat sides came from a stored user id that Google sign-in never set,
so every message looked like the other player's; the server now marks each
message mine and the client reads its id from the token (core/me.js). Chat
rebuilt (my bubbles inline-end, grouping, times, seen, typing, reactions,
quick replies), live delivery fixed, groups fully working (join by code or
link, admins, unread, owner hand-over), presence, dock/bell/friend badges,
friend notifications, news feed, invite links, profile actions.
tests/play/social.mjs 106/106.

Challenges: the invite was only seen on the Friends screen; a global prompt
(play/invite-watch.js) now shows it everywhere with a countdown. Chess
declines failed on the result enum; the handshake lives in match_invites
with conditional updates, expiry and challenger presence. VS: self vs self
came from the stored id and profile.php answering with the caller when the
id was missing; every player's card now travels with the hand-off.
Backgammon simultaneous searches and ludo table sizes fixed.
tests/play/challenge.mjs 72/72 (4 games x 6 scenarios x 3 rounds).

Resume: leaving the screen dropped the saved match; your own sleep was
blamed on the opponent; heartbeats raced moves inside game_state (now
match_presence); moves on a bad line were dropped (outbox) and late retries
rolled boards back (stale steps refused); frozen loops restart on return.
Emotes appear at the sender's plate on both screens, per seat in ludo.
Resume and emote scenarios pass in all four suites.

Migrations applied: social, match-invites, match-presence.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>