fix(site-import): drop repeated site-wide inline rules after the first page

[tommy230]

Summary

Many sites print the same <style> block into every page's head. A site import added one copy of those rules per page to the global sheet. The later copies then beat a page's own override at equal specificity, so that page's override lost.

<!-- every page -->   <style>.nav a{color:#fff}</style>
<!-- one page only --> <style>.menu a{color:#1e1e1e}</style>

In a browser that page's link is near-black. After import, later pages' white copies land after the override, so it turns white.

The fix:

• Keeps the first copy of a rule and drops byte-identical repeats from later pages' inline CSS.
• Leaves linked stylesheets alone, and keeps repeats within one page.
• Keys rules with relative url() by page directory, so the same text in different folders stays distinct.

This assumes a page prints the site-wide block before its own rules, as themes do. A page that prints its own rule before the site-wide block would now have that rule win, where a browser lets the site-wide block win. It does not help when a page overrides the exact same selector, since the importer merges those into one rule that no longer matches the site-wide copy.

Verification

• bun run build
• bun run lint
• bun test: 7058 pass, 0 fail
• Docker/deployment check, if relevant: not relevant

New tests in src/__tests__/siteImport/inlineRuleDedupe.test.ts fail on main and pass with this change.

Checklist

• Tests cover behavior changes.
• Docs were updated when behavior, config, deployment, or public surfaces changed.
• No compatibility shim was added for old pre-release behavior.
• No secrets, local databases, uploads, or generated artifacts are included.

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