Improve subscription UI with large tier buttons

- Replace dropdown tier selection with attractive visual buttons
- Add tier-button CSS with hover effects and selection states
- Remove 'or pay by card' divider from subscription form for cleaner UI
- Update JavaScript to handle tier button selection events
- Fix Stripe module import conflict by renaming stripe directory to stripe_config
- Add responsive grid layout for tier buttons on mobile devices

stripe_config/.env.example

@@ -0,0 +1,3 @@
+STRIPE_SECRET_KEY=sk_test_yourkeyhere
+STRIPE_PUBLISHABLE_KEY=pk_test_yourkeyhere
+DEFAULT_CURRENCY=GBP