Add environment checks to prepare-release target

[jkowall]

This PR adds a target to the that validates the Node.js version and ensures exists before running . This addresses issues where running the release preparation in an unconfigured environment causes problems.

[Copilot]

Pull request overview

Adds an environment validation step before running the release-preparation automation to reduce failures in unconfigured local environments.

Changes:

• Introduce a check-env Makefile target to validate Node.js version (via .nvmrc) and the presence of node_modules.
• Make prepare-release depend on check-env.
• Update release artifacts (CHANGELOG.md and packages/jaeger-ui/package.json version).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
Makefile	Adds check-env target and wires it into prepare-release.
packages/jaeger-ui/package.json	Bumps workspace version to 2.15.0.
CHANGELOG.md	Adds a v2.15.0 section with release notes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.21%. Comparing base (6f319e2) to head (f315274).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3512   +/-   ##
=======================================
  Coverage   89.21%   89.21%           
=======================================
  Files         330      330           
  Lines        9889     9889           
  Branches     2568     2568           
=======================================
  Hits         8822     8822           
  Misses        925      925           
  Partials      142      142           

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[jkowall]

Addressed your feedback — simplified check-env to just run nvm use instead of the manual version comparison. Also rebased on latest main and dropped the unrelated version bump/CHANGELOG commits so this PR is now Makefile-only.