feat: Add Pro features discovery banner

Adds a dismissible banner in the topbar that informs users about
PLANKA Pro features.

- Dismissible per-user (stored in localStorage)
- Reappears after 30 days
- Rotates between main message and 3 feature highlights
- Links to planka.app/pro (with ref parameter for anonymous source attribution)

client/src/locales/en-US/core.js

@@ -140,6 +140,13 @@ export default {
       customFields_title: 'Custom Fields',
       customerPanel_title: 'Customer Panel',
       dangerZone_title: 'Danger Zone',
+      discoverPlankaPro: '✨ More features for your boards: Discover PLANKA Pro',
+      discoverPlankaPro_title: 'Discover PLANKA Pro',
+      dismissProBannerFor30Days: 'Dismiss for 30 days',
+      upgradeTeamToPro_title: 'Upgrade Team to Pro',
+      proFeatureCalendar: '✨ Calendar View for your boards',
+      proFeatureRecurringCards: '✨ Recurring Cards',
+      proFeatureGuestRoles: '✨ Guest Roles & External Collaboration',
       date: 'Date',
       deactivateUser_title: 'Deactivate User',
       defaultCardType_title: 'Default Card Type',