fix(android): keep the toolbar out of the status and navigation bars (Issue #50)

NativeActivity draws the whole window, also below the system bars, so the
top toolbar button lay under the status bar and taps there went to
Android (no other notebook could be opened). InkActivity now reports the
system bar and cutout insets; the app shrinks egui's screen rect to the
free area and adds the side insets to the page centering. The page itself
still scrolls below the bars.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui.rs

@@ -497,8 +497,9 @@ pub struct UiState {
     pub scrollbar_grab: Option<f32>,
     /// Aktuelle Abbildung Seite ↔ Bildschirm (für Lineal und Treffertests)
     pub page_view: Option<PageView>,
-    /// Fenstergröße in logischen Punkten (vom letzten Bild)
-    pub screen_size: egui::Vec2,
+    /// Usable part of the window in logical points (from the last frame; on Android without the
+    /// system bars)
+    pub screen_rect: egui::Rect,
     /// Eingabefeld „Titel dieser Seite“ in der Seitenübersicht und die Seite, zu der es gehört
     pub page_title_input: String,
     pub page_title_for: Option<u64>,
@@ -593,7 +594,7 @@ impl Default for UiState {
             ruler_drag: None,
             scrollbar_grab: None,
             page_view: None,
-            screen_size: egui::Vec2::ZERO,
+            screen_rect: egui::Rect::ZERO,
             page_title_input: String::new(),
             page_title_for: None,
             page_picker_list: false,
@@ -666,7 +667,7 @@ impl UiState {
 
     /// Mitte des Bereichs neben der Werkzeugleiste (logische Punkte).
     fn visible_canvas_center(&self) -> egui::Pos2 {
-        (self.screen_size / 2.0).to_pos2()
+        self.screen_rect.center()
             + egui::vec2(
                 match self.sidebar_side {
                     SidebarSide::Left => self.sidebar_reserved_width() / 2.0,
@@ -3200,7 +3201,7 @@ pub fn render_ui(
     updater: &crate::updater::UpdaterController,
 ) -> Vec<UiAction> {
     let mut actions = std::mem::take(&mut state.queued_actions);
-    state.screen_size = ctx.screen_rect().size();
+    state.screen_rect = ctx.screen_rect();
     page_separators(ctx, state, notebook);
     page_scrollbar(ctx, state, notebook, &mut actions);
     space_preview(ctx, state, notebook);