fix: make Windows HTML preview capture exact viewport

[henryZhouLikeStudy]

Summary

• Fix Windows NativeAOT CDP screenshot failures caused by reflection-based JSON serialization.
• Use deterministic DevTools target creation with bounded fallback and CDP timeouts.
• Prevent browser subprocess pipe deadlocks and preserve exact viewport capture.
• Normalize headless single-slide canvas sizing without changing grid/multi-slide behavior.
• Keep platform build jobs independent when macOS signing is unavailable.

Validation

• Validated with a generated synthetic PPTX fixture containing only placeholder title, footer, and legend text; no business or user documents were used.
• Synthetic 1280x720 screenshot completed successfully with the footer and legend visible to the bottom edge.
• GitHub Actions Windows x64 smoke/build validation is enabled for this branch.
• No screenshots, PPTX files, generated previews, or user-provided files are included in this PR.

No release was created.