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.

frontend/src/components/EmojiPicker.tsx

@@ -0,0 +1,43 @@
+import { useEscapeKey } from '../hooks/useEscapeKey'
+import { EMOJI_CATEGORIES } from '../lib/emoji'
+import './EmojiPicker.css'
+
+interface EmojiPickerProps {
+  onPick: (emoji: string) => void
+  onClose: () => void
+  placement?: 'above' | 'below'
+  align?: 'left' | 'right'
+}
+
+export function EmojiPicker({ onPick, onClose, placement = 'below', align = 'left' }: EmojiPickerProps) {
+  useEscapeKey(onClose)
+
+  return (
+    <>
+      <div className="emoji-picker-scrim" onClick={onClose} />
+      <div
+        className={`emoji-picker emoji-picker-${placement} emoji-picker-${align}`}
+        role="menu"
+      >
+        {EMOJI_CATEGORIES.map((category) => (
+          <div key={category.label} className="emoji-picker-category">
+            <div className="emoji-picker-category-label">{category.label}</div>
+            <div className="emoji-picker-grid">
+              {category.emoji.map((emoji) => (
+                <button
+                  key={emoji}
+                  type="button"
+                  role="menuitem"
+                  className="emoji-picker-item"
+                  onClick={() => onPick(emoji)}
+                >
+                  {emoji}
+                </button>
+              ))}
+            </div>
+          </div>
+        ))}
+      </div>
+    </>
+  )
+}