Add fullscreen controls to Mermaid diagrams

[filipdanisko]

Supersedes #16.

Original implementation by @davidmoshal. Commit af7ff5265f21f8d75eff6f5f557da91b7e8753c7 is retained unchanged to preserve authorship.

Summary

• Open rendered Mermaid diagrams in an accessible viewport-filling dialog.
• Add toolbar, wheel, drag, pinch, and keyboard zoom/pan controls, including Fit and 100%.
• Restore the diagram and trigger focus on close.
• Keep the local viewer and interactive HTML exports identical through shared Vite-built assets.
• Preserve Mermaid strict security mode and invalid-diagram fallback behavior.
• Add permanent Chromium E2E coverage and update viewer documentation.

Main integration

• Merge current main into the original contribution.
• Preserve the current viewer grid, OKF 0.2 surfaces, and related documentation.
• Regenerate viewer.js and viewer.css from the merged source.
• Verify that rebuilding the viewer leaves both committed assets bit-for-bit unchanged.
• Include no feature-specific dependency or lockfile changes.

Validation

• pnpm build:web
• pnpm test:web — 8/8
• pnpm test:cli
• pnpm test:browser — 9/9
• go run ./packages/cli/cmd/openknowledge validate Wiki
• Repeated pnpm --filter @openknowledge-sh/web build:viewer with a clean Git diff