feat(export): trim pages to visible content

[nmrtist]

Summary

Add opt-in trimming of exported PlotX pages to visible content.

• Add a sticky Trim page to visible content option to the export dialog.
• Support PNG, JPEG, TIFF, SVG, and PDF while preserving format-specific 1 px / 1 pt safety padding.
• Compute bitmap bounds from final lossless RGBA pixels before encoding.
• Compute SVG bounds from clip-aware rendered geometry while excluding the page background and visually redundant figure backgrounds.
• Generate PDF pages from trimmed SVGs, including distinct MediaBox sizes for multi-page exports.
• Preserve empty pages at their authored size and apply journal preset scaling without refitting cropped content.
• Propagate raster crop and allocation failures through the existing user-visible export error path.
• Include follow-up correctness fixes for axis simplification, minimum-cell spacing invariants, shared tiling direction logic, and tile-preview cache identity.

Project and settings schema versions remain unchanged.

Validation

• cargo pr-check — passed
• npm run build from docs/ — passed
• git diff --check — passed