Phase 5: detail screens (scrollable lists + back button) + main controller

- weather_detail: scrollable hourly forecast (time, temp, description,
  rain probability), data logic ported from old weather_detail_screen.cpp
- calendar_detail: scrollable list of all 10 events (summary + formatted
  date/all_day), ported from old calendar_detail_screen.cpp
- mvg: scrollable departures list (line badge U/S colored, destination,
  time/delay, cancellation), 1-minute refresh timer ported from old
  mvg_screen.cpp
- all three use back_button widget (fixed bottom-left)
- main.cpp: Screen controller, navigates between screens via callbacks,
  5-minute inactivity timeout -> home, per-screen tick() for refresh

src/ui/calendar_detail_screen.cpp

@@ -0,0 +1,77 @@
+#include "calendar_detail_screen.h"
+#include "ui/widgets/back_button.h"
+#include "api/calendar_api.h"
+#include <theme.h>
+#include <text_utils.h>
+#include <date_utils.h>
+#include <lvgl.h>
+
+namespace {
+
+ScreenId (*g_navigate)(ScreenId) = nullptr;
+void on_back(lv_event_t *) { if (g_navigate) g_navigate(ScreenId::HOME); }
+
+CalendarData lastCalendar;
+bool ok = false;
+lv_obj_t *list = nullptr;
+
+void buildList() {
+    if (!list) return;
+    lv_obj_clean(list);
+
+    if (!ok) {
+        lv_obj_t *lbl = lv_label_create(list);
+        lv_label_set_text(lbl, LV_SYMBOL_WARNING " Keine Verbindung");
+        lv_obj_set_style_text_color(lbl, Theme::accentError(), 0);
+        return;
+    }
+
+    for (const auto &ev : lastCalendar.events) {
+        lv_obj_t *row = lv_obj_create(list);
+        lv_obj_set_size(row, 280, LV_SIZE_CONTENT);
+        lv_obj_set_style_bg_opa(row, LV_OPA_TRANSP, 0);
+        lv_obj_set_style_border_width(row, 0, 0);
+        lv_obj_set_style_pad_all(row, 6, 0);
+        lv_obj_clear_flag(row, LV_OBJ_FLAG_SCROLLABLE);
+
+        lv_obj_t *sum = lv_label_create(row);
+        lv_label_set_text(sum, sanitizeGermanText(ev.summary.substring(0, 30)).c_str());
+        lv_obj_set_style_text_color(sum, Theme::text(), 0);
+        lv_obj_align(sum, LV_ALIGN_TOP_LEFT, 0, 0);
+
+        lv_obj_t *when = lv_label_create(row);
+        lv_label_set_text(when, DateUtils::formatShortDE(ev.startAt, ev.allDay).c_str());
+        lv_obj_set_style_text_color(when, Theme::accentCalendar(), 0);
+        lv_obj_align(when, LV_ALIGN_TOP_LEFT, 0, 20);
+    }
+}
+
+} // namespace
+
+void calendarDetailScreen_setNavigator(ScreenId (*nav)(ScreenId)) { g_navigate = nav; }
+
+void calendarDetailScreen_create(Screen &s) {
+    list = lv_list_create(s.root);
+    lv_obj_set_pos(list, 0, 0);
+    lv_obj_set_size(list, 320, 480 - 80);
+    lv_obj_set_style_bg_color(list, Theme::bg(), 0);
+    lv_obj_set_style_border_width(list, 0, 0);
+    back_button_create(s.root, on_back, nullptr);
+}
+
+void calendarDetailScreen_refresh(Screen &) {
+    CalendarData c = fetchCalendar();
+    ok = c.valid;
+    if (c.valid) lastCalendar = c;
+    buildList();
+}
+
+void calendarDetailScreen_tick(Screen &) {}
+
+Screen calendarDetailScreen_make() {
+    Screen s;
+    s.create_fn  = calendarDetailScreen_create;
+    s.refresh_fn = calendarDetailScreen_refresh;
+    s.tick_fn    = calendarDetailScreen_tick;
+    return s;
+}