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/src/app.css

@@ -1,3 +1,11 @@
 @import 'tailwindcss';
 @plugin '@tailwindcss/forms';
 @plugin '@tailwindcss/typography';
+
+@theme {
+	--font-sans: 'Google Sans', 'Google Sans Text', system-ui, -apple-system, sans-serif;
+}
+
+a, button, [role="button"], input[type="submit"], select, summary {
+	cursor: pointer;
+}