fix(cli): #1709 guard frontmatter title/label/id decode so a literal % cannot crash the build

[jstockdi]

Related Issue

Resolves #1709

Documentation

N/A — no user-facing documentation changes (bug fix only).

Summary of Changes

1. Added a small safeDecodeURIComponent helper in
   packages/cli/src/lifecycles/graph.js that wraps decodeURIComponent in a
   try/catch and returns the original string on URIError.
2. Used it for the page id, label, and title decodes so valid
   percent-encoded filenames still decode, but a literal % in frontmatter
   (e.g. 100% Complete, Save 20%) can no longer crash the build.
3. Added a regression test case
   packages/cli/test/cases/build.default.frontmatter-percent/ with a fixture
   page whose frontmatter title/label contain %, asserting the build succeeds
   and the title reaches the output verbatim.
4. No breaking changes — valid percent-encoded filenames decode exactly as
   before; only the previously-fatal malformed-input path now falls back to the
   raw string.