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>

web/static/js/shortcuts.js

@@ -81,3 +81,11 @@ document.addEventListener('mouseout', (e) => {
     hideTimer = setTimeout(hideCard, 250);
   }
 });
+// click a mention (or any user-card target) to open its card immediately
+document.addEventListener('click', (e) => {
+  const target = e.target.closest('[data-user-card]');
+  if (!target) return;
+  e.preventDefault();
+  clearTimeout(hideTimer);
+  showCard(target, target.dataset.userCard);
+});