Add Workout Stats screen (daily/weekly/monthly) — KC-14

Aggregate the per-day workout logs into performance stats:
- WorkoutDayLog.volume + StatPeriod/WorkoutStat/StatBucket.
- WorkoutViewModel.stat/statInterval/statBuckets (current vs previous).
- WorkoutStatsView: Day/Week/Month selector, Workouts/Sets/Volume cards
  with %-delta vs previous period, metric chooser, and a Swift Charts bar
  trend. Reached via a chart button in the Workout header.

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

KisaniCal/Models/ExerciseModels.swift

@@ -67,6 +67,34 @@ struct WorkoutDayLog: Identifiable, Codable {
 
     var totalSets: Int { exercises.reduce(0) { $0 + $1.sets.count } }
     var doneSets:  Int { exercises.reduce(0) { $0 + $1.sets.filter(\.done).count } }
+    /// Total training volume in kg for completed sets (Σ weight × reps).
+    var volume: Double {
+        exercises.reduce(0) { sum, ex in
+            sum + ex.sets.reduce(0) { $0 + ($1.done ? $1.weight * Double($1.reps) : 0) }
+        }
+    }
+}
+
+// MARK: - Workout stats aggregation
+
+enum StatPeriod: String, CaseIterable, Identifiable {
+    case day = "Day", week = "Week", month = "Month"
+    var id: String { rawValue }
+    var calComponent: Calendar.Component {
+        switch self { case .day: return .day; case .week: return .weekOfYear; case .month: return .month }
+    }
+}
+
+struct WorkoutStat {
+    var workouts: Int = 0
+    var sets: Int = 0
+    var volume: Double = 0
+}
+
+struct StatBucket: Identifiable {
+    let id = UUID()
+    let label: String
+    let stat: WorkoutStat
 }
 
 struct LoggedExercise: Identifiable, Codable {
@@ -382,6 +410,47 @@ class WorkoutViewModel: ObservableObject {
         save()
     }
 
+    // MARK: - Stats (daily / weekly / monthly performance)
+
+    /// History logs whose date falls inside `interval`.
+    private func logs(in interval: DateInterval) -> [WorkoutDayLog] {
+        history.filter { guard let d = iso.date(from: $0.date) else { return false }
+                         return interval.contains(d) }
+    }
+
+    /// The calendar interval for `period`, shifted back by `offset` periods (0 = current).
+    func statInterval(_ period: StatPeriod, offset: Int) -> DateInterval {
+        let cal = Calendar.current
+        let anchor = cal.date(byAdding: period.calComponent, value: offset, to: Date()) ?? Date()
+        return cal.dateInterval(of: period.calComponent, for: anchor)
+            ?? DateInterval(start: cal.startOfDay(for: anchor), duration: 86400)
+    }
+
+    /// Aggregate workout stats for one period (offset 0 = current, -1 = previous).
+    func stat(_ period: StatPeriod, offset: Int = 0) -> WorkoutStat {
+        let logs = logs(in: statInterval(period, offset: offset))
+        return WorkoutStat(
+            workouts: logs.count,
+            sets:     logs.reduce(0) { $0 + $1.doneSets },
+            volume:   logs.reduce(0) { $0 + $1.volume }
+        )
+    }
+
+    /// A series of `count` recent periods, oldest → newest, for charting.
+    func statBuckets(_ period: StatPeriod, count: Int) -> [StatBucket] {
+        let cal = Calendar.current
+        let fmt = DateFormatter()
+        switch period {
+        case .day:   fmt.dateFormat = "EEE"
+        case .week:  fmt.dateFormat = "MMM d"
+        case .month: fmt.dateFormat = "MMM"
+        }
+        return (0..<count).reversed().map { back in
+            let interval = statInterval(period, offset: -back)
+            return StatBucket(label: fmt.string(from: interval.start), stat: stat(period, offset: -back))
+        }
+    }
+
     /// The program assigned to today in the weekly schedule (nil = rest day).
     var todaysScheduledProgram: WorkoutProgram? { program(for: Date()) }