fix(reader): prevent invalid previous-page curl at first page

[everpcpc]

What

• Tighten page-curl gesture begin guard in simulated page-turning mode
• When pan direction is ambiguous, start gesture only if both previous and next pages exist
• Keep existing directional checks unchanged when direction is clear
• Bump CURRENT_PROJECT_VERSION from 284 to 285

Why

At boundary pages (especially the first page), an ambiguous initial pan could still start a curl toward a non-existent previous page and crash.

Closes #489