feat: restore configurable hotfile option

[benny123tw]

Summary

This PR restores the hotfile feature that was removed in commit 588863c, addressing issue #2. The hotfile is now configurable and can be disabled by setting hotFile to false.

Why is this needed?

The hotfile allows Spring applications to detect when the Vite dev server is running. Spring can check for the existence of this file to determine whether to:

• Load assets from the Vite dev server (development mode)
• Load assets from the build directory (production mode)

Changes

• ✅ Add hotFile configuration option to VitePluginJavaConfig (string | false)
• ✅ Write hotfile on server start containing the dev server URL
• ✅ Clean up hotfile automatically on process exit
• ✅ Update .gitignore to ignore the hot file
• ✅ Update README with hotfile documentation and examples

Configuration

The hotfile is enabled by default at public/hot, but can be customized or disabled:

```typescript
java({
input: 'src/main.ts',
hotFile: 'static/hot', // Custom path
// or
hotFile: false, // Disable hotfile generation
})
```

Test Plan

• Lint passes
• Type checks pass
• All tests pass
• Feature tested manually

Closes #2

🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

This PR restores the hotfile feature for the Vite Java plugin, allowing Spring applications to detect when the Vite dev server is running by checking for a hotfile containing the server URL.

• Adds configurable hotFile option that can be set to a custom path or disabled with false
• Automatically writes hotfile on server start and cleans it up on process exit
• Updates documentation with hotfile configuration examples

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
packages/vite-plugin-java/src/vite-plugin-java.ts	Implements hotfile writing on server start and cleanup on exit
packages/vite-plugin-java/src/index.ts	Adds hotFile configuration option to the interface
packages/vite-plugin-java/README.md	Documents the new hotFile configuration option with examples

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[Copilot]

The cleanup function should also be called for the other process exit handlers to ensure the hotfile is cleaned up in all exit scenarios.

Suggested change

	process.on('SIGINT', () => process.exit())
	process.on('SIGTERM', () => process.exit())
	process.on('SIGHUP', () => process.exit())
	process.on('SIGINT', () => { clean(); process.exit() })
	process.on('SIGTERM', () => { clean(); process.exit() })
	process.on('SIGHUP', () => { clean(); process.exit() })

[Copilot]

These signal handlers should call the cleanup function before exiting to ensure the hotfile is removed when the process is terminated by signals.

Suggested change

	process.on('SIGINT', () => process.exit())
	process.on('SIGTERM', () => process.exit())
	process.on('SIGHUP', () => process.exit())
	process.on('SIGINT', () => { clean(); process.exit() })
	process.on('SIGTERM', () => { clean(); process.exit() })
	process.on('SIGHUP', () => { clean(); process.exit() })