fix(input): remember "draw with the finger", on by default on Android (Issue #50)

The setting was never saved and always started off, so one finger only
moved the page after every start. On a phone without a pen nothing could
be written until it was switched on again, and an update restarts the
app. It is now saved with the other UI settings and starts on on
Android, where phones usually have no pen.

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

src/settings.rs

@@ -234,6 +234,9 @@ pub struct UiSettings {
     pub storage_dir: Option<PathBuf>,
     /// Compute handwritten math after `=` (Issue #46)
     pub math_assist: bool,
+    /// One finger draws instead of moving the page. On by default on Android, where phones
+    /// usually have no pen (Issue #50)
+    pub touch_draw: bool,
 }
 
 impl Default for UiSettings {
@@ -256,6 +259,7 @@ impl Default for UiSettings {
             eraser_radius_mm: 3.0,
             storage_dir: None,
             math_assist: false,
+            touch_draw: cfg!(target_os = "android"),
         }
     }
 }
@@ -354,6 +358,7 @@ mod tests {
             eraser_radius_mm: 6.0,
             storage_dir: Some(PathBuf::from("/home/pia/Nextcloud/ink-notes")),
             math_assist: true,
+            touch_draw: true,
         };
         settings.save_to(&path).unwrap();
         assert_eq!(UiSettings::load_from(&path), settings);