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/chat-widget.js

@@ -3,7 +3,7 @@
 // plain-text composer. Reuses the conversations API + live WS channel.
 import { api } from '/static/js/api.js';
 import { subscribe, onPollFallback } from '/static/js/ws.js';
-import { attachMentions } from '/static/js/mention.js';
+import { attachMentions, mentionHTML } from '/static/js/mention.js';
 
 if (document.body.dataset.loggedIn === '1' && location.pathname !== '/messages') {
   let meId = '';
@@ -113,7 +113,7 @@ if (document.body.dataset.loggedIn === '1' && location.pathname !== '/messages')
     input.value = '';
     try {
       await api('POST', `/api/v1/conversations/${current.id}/messages`, {
-        body: '<p>' + esc(text) + '</p>',
+        body: '<p>' + mentionHTML(text, input) + '</p>',
       });
     } catch (e) { input.value = text; }
   }