Add monorepo GitHub Actions CI with separate backend and frontend jobs

[FencyJay]

Motivation

• Consolidate CI for the monorepo to run backend and frontend checks independently.
• Add support for the work branch and manual triggering via workflow_dispatch, and modernize action versions.
• Surface CI behavior in the project documentation by adding a CI section to README.md.

Description

• Rename the workflow to Monorepo CI and add triggers for pushes and PRs to master and work, plus workflow_dispatch for manual runs.
• Split the single job into backend and frontend jobs and set defaults.working-directory to coder-practice-backend and coder-practice-frontend respectively.
• Upgrade actions to actions/checkout@v4, actions/setup-java@v4 with Maven caching, and add actions/setup-node@v4 with npm caching and node-version: '20'.
• Configure the backend to run mvn -B test and the frontend to run npm ci, npm run lint, and npm run build, and add a CI explanation to README.md.

Testing

• No automated CI runs were executed as part of this configuration change.
• The added workflow is configured to run backend tests via mvn -B test and frontend checks via npm ci, npm run lint, and npm run build when triggered.

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Codex Task

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Project	Deployment	Actions	Updated (UTC)
code-practice-frontend	Ready	Preview, Comment	Mar 5, 2026 6:00am

[FencyJay]

有问题，不提交