feat: Add CSS chunking support and unify CSS file format

[kinboyw]

Summary

This PR implements CSS file chunking for large files and unifies the CSS data structure to use an array format for both single files and multiple chunks.

Changes

• ✅ Add splitCssIntoChunks() method to split large CSS files at rule boundaries (>500KB threshold)
• ✅ Unify CSS data structure: use array format Array<{ url: string, hash: string }> for both single files and multiple chunks
• ✅ Fix CSS byte length calculation using TextEncoder for accurate UTF-8 encoding (fixes ERR_CONTENT_LENGTH_MISMATCH)
• ✅ Generate unique hash for each chunk (includes chunk index) to ensure different filenames on server
• ✅ Add backward compatibility: auto-convert old CSS format to new array format
• ✅ Fix empty array check in processCss() to properly handle empty cssResult arrays

Files modified:

• src/api.ts: Update CheckFilesResult interface to use array format
• src/NoteTemplate.ts: Update css field to use array format
• src/note.ts: Add chunking logic and update CSS processing

Benefits

• 🚀 Large CSS files are automatically split into smaller chunks, improving page load performance
• 🔧 Unified array format simplifies server-side processing
• ✅ Each chunk has unique hash for proper server-side validation
• 🔄 Backward compatible with existing shared notes

Testing

• Tested with large CSS files (>500KB) - verified chunking works correctly
• Tested backward compatibility with existing shared notes
• Verified CSS chunks have unique URLs and hashes

Related Issues

Fixes issues with:

• Large CSS files blocking page load
• ERR_CONTENT_LENGTH_MISMATCH for large CSS files

Technical Details

CSS Chunking Algorithm

• Splits CSS at rule boundaries (after }) to avoid breaking CSS syntax
• Uses TextEncoder for accurate byte size calculation
• Includes fallback to force split if a chunk becomes excessively large (1.5x maxChunkSize)
• Each chunk gets a unique hash: sha1(chunkIndex-chunkContentHash-totalChunks)