feat: make month week start configurable

Add a Monday-default --week-start option for every weekday and carry it through domain layout, full and compact rendering, event query windows, print mode, and the interactive TUI.

Preserve byte-compatible Monday output and cover rotated headers, spill dates, invalid input, sanitizer behavior, and live terminal restoration.

include/nocal/domain/date.hpp

@@ -31,6 +31,8 @@ using YearMonth = std::chrono::year_month;
 
 [[nodiscard]] Date first_day_of_month(YearMonth month);
 [[nodiscard]] YearMonth following_month(YearMonth month);
+[[nodiscard]] unsigned weekday_index(Date date,
+                                     std::chrono::weekday week_start);
 [[nodiscard]] unsigned monday_first_weekday(Date date);
 
 } // namespace nocal

include/nocal/domain/month_layout.hpp

@@ -20,6 +20,7 @@ struct MonthLayout {
     static constexpr std::size_t cell_count = rows * columns;
 
     YearMonth month{std::chrono::year{1970} / std::chrono::January};
+    std::chrono::weekday week_start{std::chrono::Monday};
     std::array<MonthCell, cell_count> cells{};
 
     [[nodiscard]] const MonthCell& at(std::size_t row, std::size_t column) const;
@@ -28,6 +29,7 @@ struct MonthLayout {
     [[nodiscard]] Date last_visible_date() const noexcept;
 };
 
-[[nodiscard]] MonthLayout make_month_layout(YearMonth month);
+[[nodiscard]] MonthLayout make_month_layout(
+    YearMonth month, std::chrono::weekday week_start = std::chrono::Monday);
 
 } // namespace nocal

include/nocal/tui/Screen.hpp

@@ -89,6 +89,7 @@ struct ScreenState {
         std::chrono::year{1970}, std::chrono::month{1}, std::chrono::day{1}};
     std::vector<AgendaItem> agenda_items;
     std::size_t agenda_index{0};
+    std::chrono::weekday week_start{std::chrono::Monday};
 };
 
 // Render a complete frame. The result contains no cursor positioning, which

include/nocal/tui/TuiApp.hpp

@@ -26,8 +26,12 @@ public:
     using SaveCallback = std::function<void(std::span<const Event>)>;
 
     explicit TuiApp(std::vector<Event>& events, int input_fd = 0, int output_fd = 1);
+    TuiApp(std::vector<Event>& events, std::chrono::weekday week_start,
+           int input_fd = 0, int output_fd = 1);
     TuiApp(std::vector<Event>& events, SaveCallback saver, int input_fd = 0,
            int output_fd = 1);
+    TuiApp(std::vector<Event>& events, SaveCallback saver,
+           std::chrono::weekday week_start, int input_fd = 0, int output_fd = 1);
 
     [[nodiscard]] ExitReason run();
     void dispatch(Action action);