feat: Polar license key integration and dashboard UI improvements

- Add license activation flow: server validates/activates keys via Polar SDK
- Auto-activate from Polar checkout redirect (checkout_id in URL)
- Gate dashboard behind license activation (redirect to /activate if no plan)
- Preserve redirect URL through login flow for post-purchase activation
- Show plan status badge in sidebar and overview page
- Add account section to settings (email, plan, license key)
- Add svelte-sonner toasts with custom black theme and iconify icons
- Improve validation messages across all forms
- Update API key prefix to droidclaw_
- Add cursor pointer globally for clickable elements
- Support Polar sandbox mode via POLAR_SANDBOX env flag

web/drizzle/0003_familiar_green_goblin.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "user" ADD COLUMN "plan" text;--> statement-breakpoint
+ALTER TABLE "user" ADD COLUMN "polar_license_key" text;--> statement-breakpoint
+ALTER TABLE "user" ADD COLUMN "polar_customer_id" text;