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/MessageList.css

@@ -76,11 +76,20 @@
   font-size: 0.78rem;
 }
 
-.message-edit-link {
+.message-row-actions {
   display: none;
   position: absolute;
   top: 2px;
   right: var(--sp-2);
+  align-items: center;
+  gap: 4px;
+}
+
+.message-row:hover .message-row-actions {
+  display: flex;
+}
+
+.message-edit-link {
   background: var(--ds-surface-2);
   border: 1px solid var(--ds-border);
   color: var(--ds-muted);
@@ -95,8 +104,52 @@
   border-color: var(--ds-accent);
 }
 
-.message-row:hover .message-edit-link {
-  display: inline;
+.message-reaction-wrap {
+  position: relative;
+}
+
+.message-reaction-trigger {
+  background: var(--ds-surface-2);
+  border: 1px solid var(--ds-border);
+  color: var(--ds-muted);
+  font-size: 0.78rem;
+  line-height: 1;
+  cursor: pointer;
+  padding: 2px 6px;
+  border-radius: 6px;
+}
+
+.message-reaction-trigger:hover {
+  border-color: var(--ds-accent);
+}
+
+.message-reaction-pills {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px;
+  margin-top: 4px;
+}
+
+.message-reaction-pill {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  background: var(--ds-surface-2);
+  border: 1px solid var(--ds-border);
+  border-radius: var(--radius-pill);
+  color: var(--ds-muted);
+  font-size: 0.76rem;
+  padding: 2px 8px;
+  cursor: pointer;
+}
+
+.message-reaction-pill:hover {
+  border-color: var(--ds-accent);
+}
+
+.message-reaction-pill-mine {
+  border-color: var(--ds-accent);
+  color: var(--ds-text);
 }
 
 .message-edit-input {