build(md3): resolve ESM imports during compilation

[rdlabo]

The MD3 theme's emitted ESM contained extensionless imports, so direct Node imports could fail. Keep TypeScript source imports extensionless, matching Ionic's source style, and resolve them automatically during the normal build.

• Use @rdlabo/ionic-theme-utils@0.1.2 for the shared rdlabo-build-theme and rdlabo-check-esm CLIs.
• Retain tsc --noEmit for type checking and use tsdown to emit ESM JavaScript and declarations with resolved relative specifiers. Source files are unchanged; dependencies stay external and CSS output is preserved.
• Declare "type": "module", use TypeScript's bundler resolution mode, and rename the CommonJS Prettier configuration to .cjs.
• Add packed ESM and declaration checks to CI and document the shared build commands.

This follows the build-time approach discussed in rdlabo-dev/ionic-theme-ios27#222. Shared tooling: rdlabo-dev/ionic-theme-utils#5.

Validation on Node 24.17.0:

• Full package build and formatting checks.
• npm tarball imports and relative JavaScript/declaration references.
• NodeNext consumer type checking without skipLibCheck.
• Angular demo production build and lint.
• Demo unit tests: 36 passed.

Browser visual and native-device tests were not run locally for this build change.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[github-actions]

Playwright test results

 82 passed

Details

 82 tests across 2 suites
 1 minute, 36 seconds
 20e9140
 This detailed result covers Ionic 9 only. Ionic 8 runs against the same screenshots in a separate matrix job; check the workflow run for both results. To update the screenshots, comment with /update-screenshots.

[github-actions]

📊 Ionic 9 Playwright Test Report

View the detailed Ionic 9 report: https://rdlabo-dev.github.io/ionic-theme-md3/pr-50/

Ionic 8 runs against the same screenshots in a separate matrix job. View both results in the workflow run.