fix(healthkit): use let for interval captured across async let tasks

Build error (Xcode, Swift 6): 'interval' was a var built via two statements
then captured by 3 concurrent async let tasks in dailyReference — Swift 6
requires captured values to be immutable. It was never mutated after setup;
switched to a single-expression let (DateComponents(day: 1)).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

KisaniCal/Managers/HealthKitManager.swift

@@ -168,7 +168,7 @@ final class HealthKitManager: ObservableObject {
     func dailyReference(from start: Date, to end: Date) async -> [String: (steps: Int?, calories: Int?, restingHR: Int?)] {
         guard authorized, isAvailable else { return [:] }
         let anchor = Calendar.current.startOfDay(for: start)
-        var interval = DateComponents(); interval.day = 1
+        let interval = DateComponents(day: 1)
 
         async let steps = statsCollection(.stepCount, unit: .count(), options: .cumulativeSum, anchor: anchor, end: end, interval: interval)
         async let cals  = statsCollection(.activeEnergyBurned, unit: .kilocalorie(), options: .cumulativeSum, anchor: anchor, end: end, interval: interval)