Phase 3: reusable UI widgets (tile_button, back_button) + screen_base lifecycle

- tile_button: large touch tile with accent bg + title + icon + value,
  fires LV_EVENT_CLICKED (for home screen tiles)
- back_button: fixed bottom-left Zurueck button (LV_SYMBOL_LEFT),
  large touch target, reused on all detail screens
- screen_base.h: Screen struct with create/show/hide/refresh/tick
  lifecycle around an lv_obj_t root (replaces old ScreenId-only header)
- lv_conf.h: enable Montserrat 16/20/24 fonts used by the widgets

src/ui/screen_base.h

@@ -0,0 +1,53 @@
+#pragma once
+#include <lvgl.h>
+
+enum class ScreenId {
+    HOME,
+    WEATHER_DETAIL,
+    CALENDAR_DETAIL,
+    MVG
+};
+
+// Common lifecycle for all screens. Each screen owns one lv_obj_t screen
+// object; create() builds the UI once, show()/hide() toggle visibility,
+// refresh() re-fetches data and updates the widgets, tick() is called
+// every loop iteration for periodic refresh checks.
+struct Screen {
+    lv_obj_t *root = nullptr;
+    bool created = false;
+
+    // Build the screen object tree (hidden). Called once at startup.
+    void (*create_fn)(Screen &) = nullptr;
+    // Re-fetch data and update widgets.
+    void (*refresh_fn)(Screen &) = nullptr;
+    // Per-loop tick: check refresh timers / inactivity.
+    void (*tick_fn)(Screen &) = nullptr;
+
+    void create() {
+        if (created || !create_fn) return;
+        root = lv_obj_create(nullptr);
+        lv_obj_set_size(root, 320, 480);
+        lv_obj_set_style_bg_color(root, lv_color_hex(0x000000), 0);
+        lv_obj_set_style_border_width(root, 0, 0);
+        lv_obj_set_style_pad_all(root, 0, 0);
+        lv_obj_clear_flag(root, LV_OBJ_FLAG_SCROLLABLE);
+        create_fn(*this);
+        created = true;
+    }
+
+    void show() {
+        if (!created) create();
+        lv_screen_load(root);
+        refresh();
+    }
+
+    void hide() {}
+
+    void refresh() {
+        if (created && refresh_fn) refresh_fn(*this);
+    }
+
+    void tick() {
+        if (created && tick_fn) tick_fn(*this);
+    }
+};