Initiales HabitTracker-Projekt: SwiftUI + SwiftData Gewohnheiten-Tracker

Natives iOS-App-Gerüst (Xcode 26, synchronisierte Ordner, iOS 18+).

Features:
- Gewohnheiten anlegen (Name, SF-Symbol, Farbe), heute abhaken, Streaks, Löschen
- Detailansicht mit Monatskalender (Tage nachtragbar) und Statistiken
- Tägliche Erinnerungen via lokale Notifications
- Home-Screen-Widget (klein/mittel) mit App-Group-Datenaustausch

HabitTracker/Views/MonthCalendarView.swift

@@ -0,0 +1,129 @@
+import SwiftUI
+
+struct MonthCalendarView: View {
+    let habit: Habit
+    let onToggle: (Date) -> Void
+
+    @State private var monthStart = Calendar.german.startOfMonth(for: .now)
+
+    private let weekdaySymbols = ["Mo", "Di", "Mi", "Do", "Fr", "Sa", "So"]
+    private let columns = Array(repeating: GridItem(.flexible()), count: 7)
+
+    private var calendar: Calendar { .german }
+    private var today: Date { calendar.startOfDay(for: .now) }
+
+    var body: some View {
+        VStack(spacing: 12) {
+            header
+
+            LazyVGrid(columns: columns, spacing: 8) {
+                ForEach(weekdaySymbols, id: \.self) { symbol in
+                    Text(symbol)
+                        .font(.caption2)
+                        .foregroundStyle(.secondary)
+                }
+
+                ForEach(Array(dayCells.enumerated()), id: \.offset) { _, date in
+                    if let date {
+                        dayCell(for: date)
+                    } else {
+                        Color.clear.frame(height: 36)
+                    }
+                }
+            }
+        }
+    }
+
+    private var header: some View {
+        HStack {
+            Button {
+                shiftMonth(by: -1)
+            } label: {
+                Image(systemName: "chevron.left")
+            }
+
+            Spacer()
+
+            Text(monthTitle)
+                .font(.headline)
+
+            Spacer()
+
+            Button {
+                shiftMonth(by: 1)
+            } label: {
+                Image(systemName: "chevron.right")
+            }
+            .disabled(isCurrentMonthOrLater)
+        }
+        .buttonStyle(.borderless)
+    }
+
+    private func dayCell(for date: Date) -> some View {
+        let isDone = habit.isCompleted(on: date, calendar: calendar)
+        let isToday = calendar.isDate(date, inSameDayAs: today)
+        let isFuture = date > today
+        let color = Color(hex: habit.colorHex)
+
+        return Text("\(calendar.component(.day, from: date))")
+            .font(.callout)
+            .frame(width: 36, height: 36)
+            .background {
+                if isDone {
+                    Circle().fill(color)
+                } else if isToday {
+                    Circle().strokeBorder(color, lineWidth: 1.5)
+                }
+            }
+            .foregroundStyle(isDone ? .white : (isFuture ? Color.secondary.opacity(0.4) : .primary))
+            .contentShape(Circle())
+            .onTapGesture {
+                if !isFuture { onToggle(date) }
+            }
+    }
+
+    private var monthTitle: String {
+        let formatter = DateFormatter()
+        formatter.calendar = calendar
+        formatter.locale = Locale(identifier: "de_DE")
+        formatter.dateFormat = "LLLL yyyy"
+        return formatter.string(from: monthStart)
+    }
+
+    /// Leading nil padding for the weekday offset, then one date per day in the month.
+    private var dayCells: [Date?] {
+        guard let range = calendar.range(of: .day, in: .month, for: monthStart) else { return [] }
+        let weekday = calendar.component(.weekday, from: monthStart)
+        let leadingBlanks = (weekday - calendar.firstWeekday + 7) % 7
+
+        var cells: [Date?] = Array(repeating: nil, count: leadingBlanks)
+        for day in range {
+            cells.append(calendar.date(byAdding: .day, value: day - 1, to: monthStart))
+        }
+        return cells
+    }
+
+    private var isCurrentMonthOrLater: Bool {
+        monthStart >= calendar.startOfMonth(for: .now)
+    }
+
+    private func shiftMonth(by value: Int) {
+        if let next = calendar.date(byAdding: .month, value: value, to: monthStart) {
+            monthStart = next
+        }
+    }
+}
+
+extension Calendar {
+    /// Gregorian calendar with Monday as the first weekday (German convention).
+    static var german: Calendar {
+        var calendar = Calendar(identifier: .gregorian)
+        calendar.firstWeekday = 2
+        return calendar
+    }
+
+    func startOfMonth(for date: Date) -> Date {
+        let components = dateComponents([.year, .month], from: date)
+        return self.date(from: components) ?? startOfDay(for: date)
+    }
+}