fix(sanitize): prevent double user-content- prefix on footnote ids

[helloyou2012]

Footnote and heading ids were rendered with a doubled prefix (e.g. user-content-user-content-fn-1) because both remark-rehype and rehype-sanitize default their clobberPrefix to user-content-, so
the prefix was applied twice in the pipeline.

remark-rehype emits footnote backref hrefs (href="#fn-1") coordinated with its own clobberPrefix, so disabling the prefix upstream would desync hrefs from their targets and break footnote navigation. Disable clobberPrefix on rehype-sanitize instead, letting remark-rehype own the single, consistent prefixing of both hrefs and ids.

• index.tsx: set clobberPrefix: "" on defaultSanitizeSchema
• tests/footnotes.test.tsx: add regression test asserting footnote
  ids are prefixed exactly once
• tests/allowed-tags.test.tsx: update assertions — user-provided
  ids on custom tags are no longer clobbered by sanitize

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

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Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

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Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

Additional Notes

[helloyou2012]

Hi @haydenbleasel ! This PR fixes an issue where footnote links can't be navigated to.

Problem: Footnote definition elements have their id prefixed with user-content- twice
(id="user-content-user-content-fn-1"), while the backref link's href only gets the prefix once (#user-content-fn-1).
The href and target id no longer match, so clicking a footnote goes nowhere.

Root cause: Both remark-rehype and rehype-sanitize default their clobberPrefix to user-content-, and the two stages
stack, producing a doubled prefix on ids.

Fix: Disable clobberPrefix on rehype-sanitize so remark-rehype remains the single source of prefixing, keeping ids and
hrefs in sync.

Would appreciate a review — thanks!