feat: @-mention autocomplete; consultant bounty-board access; clickable atomization tasks

- @-mentions: typing "@" in task comments, the messages composer and the chat
  widget now opens a user picker (new shared mention.js) that inserts "@Name ".
  Backed by the existing /api/v1/users search; menus de-dupe per field.
- Consultants can now open the bounty board (read-only): nav link + page/API
  access extended to consultants, board visibility resolves their assigned
  customers, and cards show "Open" instead of claim actions for non-developers.
- Atomization board task titles (roots and subtasks) link to /tasks/{id} so
  consultants can reach the detail view (comments, assignment, timeline).

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

web/templates/layout.html

@@ -15,6 +15,8 @@
         {{if .User.Roles.Developer}}
           <a href="/board" {{if eq .Active "board"}}aria-current="page"{{end}}>Bounty Board</a>
           <a href="/my-tasks" {{if eq .Active "my-tasks"}}aria-current="page"{{end}}>My Tasks</a>
+        {{else if .User.Roles.Consultant}}
+          <a href="/board" {{if eq .Active "board"}}aria-current="page"{{end}}>Bounty Board</a>
         {{end}}
         {{if .User.Roles.Consultant}}
           <a href="/consultant/board" {{if eq .Active "atomization"}}aria-current="page"{{end}}>Atomization</a>