Fix XHR domain rewriting for *.notion.site users

[shigahi]

Summary

• Fix client-side XMLHttpRequest override to use NOTION_SITE_DOMAIN instead of hardcoded www.notion.so, resolving site crashes for users whose Notion pages are hosted on *.notion.site domains
• Simplify code.ts: remove redundant variables, consolidate duplicate JS asset handlers and API proxy blocks, fix indentation

Fixes #55

Test plan

• Generate a worker script with a *.notion.site URL and verify XHR rewrites use the correct domain
• Generate a worker script with a www.notion.so URL and verify it still works correctly
• Verify JS assets (/app*.js and /_assets/*.js) are served with correct Content-Type
• Verify API endpoints (syncRecordValues, getPublicSpaceData, etc.) return rewritten responses

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