feat: screen-reader-friendly linear view (L key)

Plain-text appointment list with numbered indicators, no box-drawing
characters, and explicit labels. Reuses agenda data and navigation.
Binds to uppercase L to preserve vim-style hjkl grid navigation.

include/nocal/tui/Screen.hpp

@@ -87,6 +87,7 @@ struct ScreenState {
     bool show_calendar_picker{false};
     std::size_t calendar_index{0};
     bool show_agenda{false};
+    bool show_linear_view{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;
@@ -138,6 +139,7 @@ enum class Action {
     search,
     agenda,
     calendars,
+    linear_view,
     sync,
     quit,
 };

include/nocal/tui/TuiApp.hpp

@@ -121,9 +121,12 @@ private:
     void toggle_selected_calendar();
     void begin_agenda();
     void close_agenda();
+    void begin_linear_view();
+    void close_linear_view();
     void refresh_agenda();
     void shift_agenda(int direction);
     void choose_agenda_item();
+    void choose_linear_item();
     [[nodiscard]] HistorySnapshot capture_history_snapshot() const;
     void restore_history_snapshot(HistorySnapshot snapshot);
     void push_history(std::vector<HistoryEntry>& history, HistoryEntry entry);