Improved PDF vertical reader zoom and orientation handling (#98)

- Implemented `fitZoom` calculation to properly center and scale pages, especially in landscape orientation.
- Updated `clampValues` and animation bounds to ensure content is correctly centered when zoomed out.
- Refined double-tap-to-zoom logic to toggle between `fitZoom`, 1x, and 2.5x scales.
- Replaced manual frame-based zooming with `Animatable.animateTo` for smoother transitions.
- Added `configChanges` to `MainActivity` in `AndroidManifest.xml` to handle orientation and screen size changes manually.
- Reduced delays and improved reliability of initial page snapping in `PdfViewerScreen`.

app/src/main/AndroidManifest.xml

@@ -47,6 +47,7 @@
             android:name=".MainActivity"
             android:exported="true"
             android:theme="@style/Theme.Reader"
+            android:configChanges="orientation|screenSize|screenLayout|keyboardHidden"
             android:launchMode="singleTask">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />