perf: cache editor layout geometry and restore synchronous critical CSS

[ThisIs-Developer]

This Pull Request introduces elite performance and accessibility optimizations to completely eliminate layout thrashing, forced reflows, and unstyled layout shifts.

Key Changes:

• Global Editor Geometry & Scroll Position Caching: Isolated and cached all layout values (cachedPaddingLeft, cachedPaddingRight, cachedLineHeight, etc.) in global variables, eliminating all DOM queries inside the updateLineNumbers() typing loop.
• Inlined Default Markdown Template: Moved the default preview markdown content from /sample.md into an inlined element inside index.html. This completely eliminates the startup network fetch request, FOUC, and initial layout shifts.
• Restored Synchronous Critical CSS: Reverted critical stylesheets (Bootstrap and GitHub Markdown) to load synchronously in the , preventing the unstyled paint state and massive Cumulative Layout Shift (CLS) layout shifts.
• Explicit Image Dimensions: Added explicit width and height attributes to the Markdown Viewer logo inside the default markdown preview to eliminate layout shifts.

[cloudflare-workers-and-pages]

Deploying markdown-viewer with    Cloudflare Pages

Latest commit:	abf9f21
Status:	✅  Deploy successful!
Preview URL:	https://68524f02.markdown-viewer.pages.dev
Branch Preview URL:	https://perf-layout-reflow-optimizat.markdown-viewer.pages.dev

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Project	Deployment	Actions	Updated (UTC)
markdown-viwer	Ready	Preview, Comment	May 31, 2026 3:51pm

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