Improve reader loading, nearby-page buffering and motion controls #13

Follow-up to #9 and #10. Requirements: FR-3_1, FR-3_2.

Acceptance:

• Use the host opening loader through initial EPUB pagination, with consistent position, theme and no second preparation label.
• Expose a compact page-turn animation dropdown (Slide or None) for engines that support it.
• Disabled motion turns pages instantly via commands and swipes, including spine boundaries, while respecting system reduced motion and preserving selection and saved locations.
• Keep nearby pages prepared ahead and behind, including neighboring spine items. Prepared turns must not await extra layout or exact page counts.
• Preload nearby illustrations and retain visited pages for reversing direction.
• Ensure a slow neighbor does not block preparation in the opposite direction and immediate turns after opening are not lost.
• Validate loading, settings, navigation, selection and example/browser behavior.

Implementation: #14.