Optimize blog styling

[google-labs-jules]

Created assets/main.scss to override default Minima theme settings. Changes include:

• Increased content width to 1000px.
• Updated typography to use a modern system font stack.
• Improved styling for tables, blockquotes, and headers.
• Enhanced code block styling with dark mode contrast.
• Added centering and spacing for Mermaid diagrams.
• Added shadow and rounded corners to post images.

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PR created automatically by Jules for task 5168829220095957540 started by @donglua

[google-labs-jules]

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