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>

app/services/auth_service.py

@@ -59,6 +59,7 @@ async def login(db: AsyncSession, username: str, password: str) -> str:
 
     payload = {
         "sub": str(user.id),
+        "username": user.username,
         "role": user.role,
         "exp": datetime.now(timezone.utc) + timedelta(minutes=settings.jwt_expiry_minutes),
     }