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

@paullizer is already working on this.

Since Sep 8, 2026.

## 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: #964 (broader visualization expansion), #718 (Mermaid support), and #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).