Add PDF export for v2 UI charts, diagrams, and visualizations

[Bionic711]

Summary

Add a PDF export option for charts, diagrams, and related visualizations in the v2 UI, alongside the existing PNG export option.

User Value

PNG exports are raster images and become pixelated when users zoom in, especially on large diagrams with small labels or dense detail. A vector-preserving PDF export would let users inspect, share, and print these visualizations while keeping text and diagram details clear at higher zoom levels.

Proposed Behavior

• Offer PDF as an export format wherever the v2 UI supports exporting charts, diagrams, and related visualizations.
• Preserve vector geometry and text for vector-capable visualizations so zooming does not introduce PNG-style pixelation.
• Export the full visualization, including content outside the current viewport, with its aspect ratio, labels, and layout intact.
• Keep PNG export available for users who need an image file.

Acceptance Criteria

• Users can select PDF export for supported v2 UI charts, diagrams, and related visualizations and download a valid PDF.
• PDF exports preserve vector shapes, lines, and text where the source supports them; simply embedding the existing PNG in a PDF does not satisfy the zoom-quality requirement.
• Representative large diagrams and charts remain readable when substantially zoomed in (for example, 400%), without rasterization of otherwise vector-capable content.
• Exported content is complete and not clipped; labels, connectors, colors, and aspect ratios are preserved.
• Existing PNG export continues to work without regressions.
• Tests and validation cover PDF download, output fidelity, large visualizations, and the existing PNG path. User documentation explains the export choice and any renderer-specific limitations.

Notes

• PDF itself does not guarantee scalable output: a raster-only image placed inside a PDF retains its original resolution limitations. Evaluate the export path for each supported visualization renderer.
• Keep any new browser-side export dependencies pinned and served locally, consistent with SimpleChat's browser asset policy.
• Related: Expand data visualization tools in chat #964 (broader visualization expansion), Add support for mermaid diagrams #718 (Mermaid support), and Add Visio authoring and generated diagram assets #975 (Visio authoring/generated diagram assets). This request specifically covers PDF export and zoom fidelity in the v2 UI, rather than adding new visualization types.
• Priority: P2.
• Estimated size: M (export UI, renderer integration, and cross-format validation).