Run npm install asynchronously during apps init

[jamesbroadhead]

Changes

Start npm install in the background immediately after cloning the template, so it runs in parallel with the interactive prompts rather than blocking after all questions are answered.

• Add StartNpmInstallAsync() to kick off npm ci in a goroutine, returning a channel for the result
• In runCreate(), start async npm install in the cloned template directory right after template resolution
• After template copy + plugin application, wait for the background install and move node_modules to the project directory
• Initialize() skips npm install if node_modules already exists (from the async pre-install)
• Falls back gracefully: if async install fails, cleans up and lets Initialize() retry synchronously

Why

npm install is often the slowest step of databricks apps init. It currently runs sequentially after all interactive prompts and template copying. Since npm only needs package.json/package-lock.json (which exist in the cloned template), it can run in parallel with the user answering questions, reducing perceived wait time significantly.

Tests

• TestStartNpmInstallAsyncReturnsChannel — verifies async function returns a usable channel
• TestInitializeSkipsNpmInstallWhenNodeModulesExists — verifies Initialize skips npm when node_modules pre-exists
• TestFindTemplateProjectDir (4 subtests) — verifies template project directory detection
• All existing tests pass

[github-actions]

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Trigger:
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Inputs:

• PR number: 4600
• Commit SHA: ce33866da1619550c7ebdf27bb0439876a8db4a7

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