Fix check-in progress bar always showing empty after completing check-in

Previous logic measured elapsed days from lastCompletionDate to nextCheckInDate,
so completing a check-in today reset the bar to 0/8 days instead of showing
how far through the full monthly period we are.

New logic: the bar represents the full interval (e.g. 1 month) ending at
nextCheckInDate, starting from nextCheckInDate minus that interval.
Today (Feb 20) with next check-in Feb 28 now shows ~74% progress
(23 of 31 days elapsed since Jan 28) instead of 0%.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

PortfolioJournal/Views/Dashboard/MonthlyCheckInView.swift

@@ -31,12 +31,15 @@ struct MonthlyCheckInView: View {
     }
 
     private var checkInProgress: Double {
-        guard let last = lastCompletionDate,
-              let nextDate = nextCheckInDate else { return 1 }
-        let totalDays = Double(max(1, last.startOfDay.daysBetween(nextDate.startOfDay)))
-        guard totalDays > 0 else { return 1 }
-        let elapsedDays = Double(last.startOfDay.daysBetween(Date()))
-        return min(max(elapsedDays / totalDays, 0), 1)
+        guard let nextDate = nextCheckInDate,
+              let periodStart = Calendar.current.date(
+                byAdding: .month,
+                value: -checkInIntervalMonths,
+                to: nextDate
+              ) else { return 1 }
+        let totalDays = Double(max(1, periodStart.startOfDay.daysBetween(nextDate.startOfDay)))
+        let elapsedDays = Double(max(0, periodStart.startOfDay.daysBetween(Date().startOfDay)))
+        return min(elapsedDays / totalDays, 1)
     }
 
     private var checkInIntervalMonths: Int {