feat: structured @-mentions (notify+clickable+hover), distinct links, Ctrl-rebalance sliders

- Mentions now insert a span carrying the user id (handles names with spaces):
  the sanitizer permits <span class="mention" data-user-card="ULID">, comment
  and chat notifications resolve by id and only fire for users with access,
  and mentions render as clickable chips with the shared hover user-card.
- Messages composer inserts an atomic, non-editable mention chip so the
  trailing space no longer collapses while typing.
- Links inside chat messages and task comments are underlined/accented so they
  read as clickable.
- Atomization: hold Ctrl/Cmd while dragging an effort slider to rebalance the
  sibling coefficients proportionally (sum stays ~1.00); added a hint.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/http/messages.go

@@ -232,19 +232,21 @@ func (s *Server) handleSendMessage(w http.ResponseWriter, r *http.Request) {
 	if s.sendTo != nil {
 		s.sendTo(conv.ParticipantIDs, "chat", "message", msg)
 	}
+	// Notify mentioned users who are participants (i.e. have access §11.1).
 	plain := chat.SanitizePlain(body)
+	participants := map[string]bool{}
 	for _, pid := range conv.ParticipantIDs {
-		if pid == me.ID {
+		participants[pid] = true
+	}
+	notified := map[string]bool{}
+	for _, m := range mentionUIDRe.FindAllStringSubmatch(body, 20) {
+		uid := m[1]
+		if uid == me.ID || notified[uid] || !participants[uid] {
 			continue
 		}
-		if u, err := s.store.UserByID(r.Context(), pid); err == nil {
-			first := strings.ToLower(strings.SplitN(u.Name, " ", 2)[0])
-			lower := strings.ToLower(plain)
-			if strings.Contains(lower, "@"+strings.ToLower(u.Email)) || strings.Contains(lower, "@"+first) {
-				s.notifyUser(r.Context(), pid, "mention",
-					me.Name+" mentioned you in a chat", truncateStr(plain, 120), "/messages?c="+conv.ID)
-			}
-		}
+		notified[uid] = true
+		s.notifyUser(r.Context(), uid, "mention",
+			me.Name+" mentioned you in a chat", truncateStr(plain, 120), "/messages?c="+conv.ID)
 	}
 	writeJSON(w, http.StatusCreated, map[string]any{"message": msg})
 }