Fix sidebar username, Signals filter clarity and layout

- JWT now carries a username claim; sidebar shows "Signed in as <name>"
  instead of the bare user id (sub). Re-login required for the new claim.
- Signals: Min R:R / Min Confidence inputs reflect the effective filter —
  auto-filled from the activation gate when "Qualified only" is on, reset
  to 0 when off (no more misleading 0 while the gate is active).
- Signals layout: Run Scanner moved to its own action row (it's a job
  trigger, not a filter); qualified toggle grouped with the refinement
  filters under one Filters panel.

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

frontend/src/components/layout/Sidebar.tsx

@@ -70,7 +70,7 @@ export default function Sidebar() {
           </span>
         </div>
         {username && (
-          <p className="text-xs text-gray-500 truncate px-1">{username}</p>
+          <p className="text-xs text-gray-500 truncate px-1">Signed in as {username}</p>
         )}
         <button
           onClick={logout}