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/mvg_screen.cpp

@@ -0,0 +1,105 @@
+#include "mvg_screen.h"
+#include "ui/widgets/back_button.h"
+#include "api/departures_api.h"
+#include <theme.h>
+#include <text_utils.h>
+#include <lvgl.h>
+
+namespace {
+
+ScreenId (*g_navigate)(ScreenId) = nullptr;
+void on_back(lv_event_t *) { if (g_navigate) g_navigate(ScreenId::HOME); }
+
+DeparturesData lastData;
+bool ok = false;
+lv_obj_t *list = nullptr;
+unsigned long lastFetchMs = 0;
+const unsigned long REFRESH_INTERVAL_MS = 60UL * 1000UL;
+
+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 &dep : lastData.departures) {
+        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);
+
+        // Line badge (colored text: U red, S green).
+        String badge = String(dep.icon) + " " + dep.line;
+        lv_obj_t *ln = lv_label_create(row);
+        lv_label_set_text(ln, badge.c_str());
+        lv_obj_set_style_text_color(ln,
+            dep.type == "UBAHN" ? Theme::accentUBahn() : Theme::accentSBahn(), 0);
+        lv_obj_align(ln, LV_ALIGN_TOP_LEFT, 0, 0);
+
+        String dest = sanitizeGermanText(dep.destination.substring(0, 20));
+        lv_obj_t *d = lv_label_create(row);
+        lv_label_set_text(d, dest.c_str());
+        lv_obj_set_style_text_color(d, Theme::text(), 0);
+        lv_obj_align(d, LV_ALIGN_TOP_LEFT, 0, 18);
+
+        String right;
+        if (dep.cancelled) {
+            right = "Ausfall";
+        } else {
+            right = dep.timeStr;
+            if (dep.delayMin > 0) right += " +" + String(dep.delayMin);
+        }
+        lv_obj_t *t = lv_label_create(row);
+        lv_label_set_text(t, right.c_str());
+        lv_obj_set_style_text_color(t,
+            dep.cancelled ? Theme::accentError() : Theme::text(), 0);
+        lv_obj_align(t, LV_ALIGN_TOP_RIGHT, 0, 18);
+    }
+}
+
+void doFetch() {
+    DeparturesData d = fetchDepartures();
+    ok = d.valid;
+    if (d.valid) lastData = d;
+    lastFetchMs = millis();
+}
+
+} // namespace
+
+void mvgScreen_setNavigator(ScreenId (*nav)(ScreenId)) { g_navigate = nav; }
+
+void mvgScreen_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 mvgScreen_refresh(Screen &) {
+    doFetch();
+    buildList();
+}
+
+void mvgScreen_tick(Screen &) {
+    if (lastFetchMs == 0 || millis() - lastFetchMs >= REFRESH_INTERVAL_MS) {
+        doFetch();
+        buildList();
+    }
+}
+
+Screen mvgScreen_make() {
+    Screen s;
+    s.create_fn  = mvgScreen_create;
+    s.refresh_fn = mvgScreen_refresh;
+    s.tick_fn    = mvgScreen_tick;
+    return s;
+}