Add fullscreen controls to Mermaid diagrams

[davidmoshal]

Follow-up to #14, which shipped Mermaid rendering in v0.9.0. This clean PR supersedes #15, whose branch was mistakenly force-pushed after the original rendering work became redundant.

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, and close safely before theme rerenders.
• Keep the local viewer and interactive HTML exports identical through the shared Vite-built viewer assets.
• Preserve Mermaid strict security mode and readable invalid-diagram fallback behavior.
• Add permanent Chromium E2E coverage and update the command, exporter, and changelog documentation.

Validation

• pnpm build:web
• pnpm test:web (8/8)
• pnpm test:cli
• pnpm build:cli
• pnpm test:race
• pnpm test:browser (8/8)
• openknowledge validate Wiki
• Desktop and mobile Playwright runtime audit against a real Mermaid-containing knowledge bundle

No dependency or lockfile changes are included.

Local tooling note

The aggregate pnpm test wrapper reached the unchanged packed-npm smoke test, where pnpm 10.33 propagated a project-scoped allow-scripts setting that npm 11.19 rejects. Running the unchanged test directly with node scripts/test-packed-npm.mjs passed.