feat(staged): add full-screen diagram viewer

[matt2e]

Summary:

• Add a full-screen modal viewer for rendered markdown diagrams in notes and sessions.
• Implement D3-powered diagram pan/zoom, fit-to-bounds behavior, fullscreen navigation, and repeated double-click zoom.
• Refresh diagram styles/theme tokens and add diagram zoom coverage.

Tests:

• Pre-push hooks ran on push: crates-fmt, crates-test, crates-lint, differ-ci, staged-ci.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve Pikchr overflow in the full-screen viewer

When a Pikchr SVG relies on drawing outside its viewBox, the inline preview keeps that content visible via the existing .markdown-diagram-preview-pikchr .markdown-pikchr-svg { overflow: visible } rule, but the full-screen copy is rendered under .diagram-surface and this line forces the root SVG back to overflow: hidden. Opening those diagrams in the new viewer clips labels/strokes that are visible in the preview, so the modal should explicitly keep Pikchr SVG overflow visible or reuse the preview rule.

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