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/schemas/message.py

@@ -4,6 +4,12 @@ from datetime import datetime
 from pydantic import BaseModel, ConfigDict
 
 
+class ReactionSummary(BaseModel):
+    emoji: str
+    count: int
+    user_ids: list[str]
+
+
 class MessageRead(BaseModel):
     model_config = ConfigDict(from_attributes=True)
 
@@ -13,5 +19,6 @@ class MessageRead(BaseModel):
     username: str
     content: str | None
     image_id: uuid.UUID | None
+    reactions: list[ReactionSummary]
     created_at: datetime
     edited_at: datetime | None