fix(ui): profile page bugs - billing button, cancel button, header subtitle

- Remove billing portal button (not functional without Stripe configured)
- Show cancel subscription only when user has a real Stripe subscription ID
- Remove muted subtitle from dashboard header (was appearing as grayed out)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/app/dashboard/DashboardHeader.tsx

@@ -55,8 +55,6 @@ export function DashboardHeader() {
         {/* Page title - desktop */}
         <div className="hidden items-center gap-3 lg:flex">
           <h1 className="text-sm font-semibold text-foreground">{t('dashboard.header.title')}</h1>
-          <Separator orientation="vertical" className="h-4" />
-          <span className="text-sm text-muted-foreground">{t('dashboard.header.subtitle')}</span>
         </div>
 
         {/* Right side */}