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/css/app.css

@@ -766,6 +766,27 @@ input[type=range] { width: 100%; accent-color: var(--accent); }
   border-color: color-mix(in srgb, var(--accent) 70%, black);
 }
 
+/* ---- @-mention chips + links inside chat/comments ---- */
+.mention {
+  color: var(--accent);
+  font-weight: 600;
+  cursor: pointer;
+  border-radius: var(--radius);
+  padding: 0 2px;
+  white-space: nowrap;
+}
+.mention:hover {
+  background: color-mix(in srgb, var(--accent) 16%, transparent);
+  text-decoration: underline;
+}
+/* links in messages and comments are underlined so they read as clickable */
+.chat-msg a:not(.btn), .cw-msg a:not(.btn), .comment-body a:not(.btn) {
+  color: var(--accent);
+  text-decoration: underline;
+  text-underline-offset: 2px;
+  font-weight: 500;
+}
+
 /* ---- @-mention autocomplete menu ---- */
 .mention-menu {
   position: absolute; z-index: 80;