fix: stabilize webtoon page measurement

[everpcpc]

Problem

Webtoon pages without metadata dimensions could either render with an incorrect guessed height or require on-the-fly layout changes while the user was actively scrolling. That produced distorted rendering in the missing-metadata case and visible stutter during manual swipe scrolling.

Approach

Treat missing page dimensions as a measurement state instead of a renderable state. Webtoon now probes image metadata to obtain real pixel sizes, only displays page content once a valid size is known, and caches the resulting heights. When new measurements arrive during scrolling, layout updates are deferred until the scroll interaction finishes so drag gestures are not interrupted by item reloads.

Scope

• Add shared helpers to probe image pixel sizes and update cached webtoon page heights
• Gate webtoon page rendering on known dimensions for both iOS and macOS
• Batch deferred layout refreshes after scroll completion to reduce scroll jank

Validation

• make build-macos
• make build-ios