fix: guard viewer initialization after disposal

[PathGao]

Fixes #257.

Summary

• Mark MarkdownViewer initialization as disposed during teardown.
• Stop deferred syntax/math imports, session restore, initial file loading, and startup-path handling once disposed.
• Release listeners whose registration completed after the component was destroyed.

Validation

• node --test --import tsx scripts/viewerDisposal.test.ts
• npm run check

[PathGao]

@alecdotdev This branch has been rebased and force-pushed. It depends on the latest #254 branch; please merge #254 first, then this PR, followed by #260 and #266. Validation passed: node --test --import tsx scripts/viewerDisposal.test.ts and npm run check.