fix: display default reset date when limitResetDate is null

Fixed empty "Resets on" date display for new users:

Issue:
- Users who haven't created any conversations yet have limitResetDate = NULL
- The "Resets on" field was showing empty/blank
- This confused users about when their limit would reset

Solution:
- Updated formatResetDate() in 3 components to calculate default date
- If limitResetDate is NULL, display "1 month from now"
- This gives users a clear expectation of when limits reset

Files Updated:
- app/[locale]/subscription/page.tsx
  * formatResetDate() now returns calculated date if null
- components/subscription/usage-display.tsx
  * formatResetDate() now returns calculated date if null
- components/subscription/upgrade-modal.tsx
  * formatResetDate() now returns calculated date if null
  * Removed conditional check - always show reset date

User Experience:
- New users see "Resets on: [date one month from now]"
- Once they create their first conversation, actual reset date is set
- Consistent messaging across all subscription UI components

Note: The actual limitResetDate is set when the user creates their
first conversation (in incrementConversationCount function). This fix
only affects the UI display for users who haven't chatted yet.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/[locale]/subscription/page.tsx

@@ -164,7 +164,12 @@ export default function SubscriptionPage() {
   }
 
   const formatResetDate = (dateString: string | null) => {
-    if (!dateString) return ''
+    if (!dateString) {
+      // If no reset date set, calculate 1 month from now
+      const nextMonth = new Date()
+      nextMonth.setMonth(nextMonth.getMonth() + 1)
+      return nextMonth.toLocaleDateString(locale, { year: 'numeric', month: 'long', day: 'numeric' })
+    }
     const date = new Date(dateString)
     return date.toLocaleDateString(locale, { year: 'numeric', month: 'long', day: 'numeric' })
   }