Billing portal and plan upgrades now work reliably

Two billing fixes for existing subscribers. Opening the Billing Portal silently failed on mobile (it worked on desktop) - the link is now opened in a way phones don't block. And changing your plan (for example Pro → Business) no longer dead-ends on "redirecting to secure payment" - subscribers are now sent to the billing portal's plan switcher.

Fixes

• Billing Portal opens on mobile - previously nothing happened on phones; the portal now opens reliably
• Plan upgrades work for current subscribers - switching plans routes to the billing portal instead of failing