feat: add bounded agenda view

Add a read-only 42-day occurrence agenda with keyboard navigation, six-week paging, exact return-to-grid focus, search and calendar overlays, and bounded recurrence expansion.

Keep agenda state outside persistence, document month-only mutations, and cover empty windows, filtering, overlays, navigation, rendering, and terminal restoration.

include/nocal/tui/Screen.hpp

@@ -51,6 +51,17 @@ struct SearchResultItem {
     bool recurring{false};
 };
 
+struct AgendaItem {
+    std::string focus_id;
+    std::string title;
+    std::chrono::year_month_day day;
+    std::optional<std::chrono::minutes> time_of_day;
+    bool all_day{false};
+    std::string location;
+    std::string calendar_id;
+    bool recurring{false};
+};
+
 struct ScreenState {
     std::chrono::year_month visible_month{std::chrono::year{1970}, std::chrono::month{1}};
     std::chrono::year_month_day selected_day{std::chrono::year{1970}, std::chrono::month{1},
@@ -73,6 +84,11 @@ struct ScreenState {
     bool notification_is_error{false};
     bool show_calendar_picker{false};
     std::size_t calendar_index{0};
+    bool show_agenda{false};
+    std::chrono::year_month_day agenda_start_day{
+        std::chrono::year{1970}, std::chrono::month{1}, std::chrono::day{1}};
+    std::vector<AgendaItem> agenda_items;
+    std::size_t agenda_index{0};
 };
 
 // Render a complete frame. The result contains no cursor positioning, which
@@ -117,6 +133,7 @@ enum class Action {
     undo,
     redo,
     search,
+    agenda,
     calendars,
     quit,
 };

include/nocal/tui/TuiApp.hpp

@@ -48,6 +48,7 @@ public:
     [[nodiscard]] bool calendar_picker_active() const noexcept {
         return state_.show_calendar_picker;
     }
+    [[nodiscard]] bool agenda_active() const noexcept { return state_.show_agenda; }
     [[nodiscard]] const std::vector<Calendar>& calendars() const noexcept {
         return calendars_;
     }
@@ -89,6 +90,11 @@ private:
     void begin_calendar_picker();
     void close_calendar_picker();
     void toggle_selected_calendar();
+    void begin_agenda();
+    void close_agenda();
+    void refresh_agenda();
+    void shift_agenda(int direction);
+    void choose_agenda_item();
     [[nodiscard]] HistorySnapshot capture_history_snapshot() const;
     void restore_history_snapshot(HistorySnapshot snapshot);
     void push_history(std::vector<HistoryEntry>& history, HistoryEntry entry);