`SpringGenerateProject` can be called without SpringInitializrUI being open

[jkeresman01]

Title: SpringGenerateProject can be called without SpringInitializrUI being open

Summary

The :SpringGenerateProject command can be executed even when the Spring Initializr UI is not open. This should not be allowed since the command depends on user selections (language, build tool, dependencies, etc.) made through the UI.

Steps to Reproduce

1. Open Neovim without launching :SpringInitializr
2. Run :SpringGenerateProject
3. Observe that the command executes (likely with defaults or errors)

Actual Behavior

:SpringGenerateProject runs without the UI being open, potentially generating a project with no valid selections or causing unexpected errors.

Expected Behavior

:SpringGenerateProject should only be callable when SpringInitializrUI is open and selections have been made. If the UI is not open, the command should notify the user to open the UI first.

Proposed Solution (high level)

Add a guard check in the SpringGenerateProject command handler to verify the UI is currently open before proceeding with project generation.

Acceptance Criteria

• :SpringGenerateProject is blocked when SpringInitializrUI is not open
• User receives a clear notification to open the UI first
• Command works as expected when the UI is open