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/widgets/tile_button.h

@@ -0,0 +1,26 @@
+#pragma once
+#include <lvgl.h>
+#include <theme.h>
+
+// Reusable large touch tile for the home screen. Icon + title + optional
+// value/preview lines, accent background per category. Fires a callback on tap.
+struct TileButton {
+    lv_obj_t *btn = nullptr;
+    lv_obj_t *icon = nullptr;
+    lv_obj_t *title = nullptr;
+    lv_obj_t *value = nullptr;
+};
+
+// Create a tile button inside `parent` at (x,y) with given size and accent.
+// title: heading text (e.g. "Wetter"); on_click + user_data wired to LV_EVENT_CLICKED.
+TileButton tile_button_create(lv_obj_t *parent, lv_coord_t x, lv_coord_t y,
+                              lv_coord_t w, lv_coord_t h, lv_color_t accent,
+                              const char *title, lv_event_cb_t on_click,
+                              void *user_data);
+
+// Set the big value/preview text on the tile (may be multi-line).
+void tile_button_set_value(TileButton &t, const char *text);
+
+// Set the icon text (single glyph/symbol). Icons are rendered as text labels
+// using LVGL's built-in font until bitmap icons land in Phase 6.
+void tile_button_set_icon(TileButton &t, const char *text);