Add emoji picker and message reactions (Gitea issue #11)

Composer gains an emoji picker (static curated unicode list, insert at
cursor position) and messages gain Slack/Mattermost-style reactions:
react with any emoji, toggle off by reacting again, see who reacted via
a tooltip on each pill.

Backend: MessageReaction model (unique on message_id+user_id+emoji backs
toggle semantics), WS "reaction" envelope broadcasts the full recomputed
reaction list per message (same approach as message edits), REST message
list embeds reactions so a reload doesn't lose state that only arrived
over WS.

Frontend: shared EmojiPicker component (anchored popover, Escape/outside-
click dismiss via new useEscapeKey hook) used by both the composer and a
new hover-revealed reaction trigger on each message row.

backend/app/models/__init__.py

@@ -6,6 +6,7 @@ from app.models.invite import InviteStatus, RoomInvite
 from app.models.membership import RoomMembership, RoomRole
 from app.models.message import Message
 from app.models.message_image import MessageImage
+from app.models.message_reaction import MessageReaction
 from app.models.push_subscription import PushSubscription
 from app.models.room import Room
 from app.models.user import User
@@ -19,6 +20,7 @@ __all__ = [
     "RoomRole",
     "Message",
     "MessageImage",
+    "MessageReaction",
     "RoomInvite",
     "InviteStatus",
     "PushSubscription",