Add sponsors system with time slider, LNbits invoices, and UX improvements

- Sponsors table, LNbits createInvoice, webhook handler
- Sponsor routes: create, homepage, list, my-ads, click, extend, check-payment
- Admin routes for sponsor management
- Frontend: SponsorForm, SponsorTimeSlider, SponsorCard, SponsorsSection
- Sponsors page, My Ads page, homepage sponsor block
- Header login dropdown with My Ads, Create Sponsor
- Transactions integration for sponsor payments
- View/click tracking
- OG meta fetch for sponsor images
- Sponsor modal spacing, invoice polling fallback
- Remove Lightning address and Category fields from sponsor form

Made-with: Cursor

frontend/.env.example

@@ -1,6 +1,6 @@
-# Backend API URL (required in dev when frontend runs on different port)
-# Leave empty if frontend is served from same origin as API
-VITE_API_URL=http://localhost:3001
+# Backend API URL. For dev: leave empty to use Vite proxy (recommended).
+# For production or when frontend is on different origin: set full URL e.g. http://localhost:3001
+# VITE_API_URL=
 
 # Nostr relays for fetching user profile metadata (comma-separated)
 VITE_NOSTR_RELAYS=wss://relay.damus.io,wss://relay.nostr.band,wss://nos.lol