frontend: project: Disable assigned namespaces in project creation

[pallava-joshi]

Summary

Fixes #4086
Prevents users from accidentally reassigning namespaces that are already part of another project when creating a new project.

Changes

• Added detection for namespaces that already have a headlamp.dev/project-id label (i.e., are assigned to an existing project)
• Disabled these namespaces in the namespace dropdown during project creation
• Users can still type a new namespace name, but cannot select one that's already assigned

Steps to Test

1. Navigate to the Projects tab on the home page
2. Create a project (e.g., "project-a") using a namespace (e.g., "default")
3. Return to the project list and click "Create Project" again
4. In the namespace dropdown, observe that "default" is now greyed out and cannot be selected
5. You can still type a new namespace name to create a new project

Screenshots (if applicable)

Screen.Recording.2026-01-25.at.6.41.12.PM.mov

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[Copilot]

Pull request overview

Prevents users from reassigning namespaces that are already associated with an existing project during “New Project” creation (fixes #4086).

Changes:

• Detect namespaces that already have the headlamp.dev/project-id label.
• Disable already-assigned namespaces in the namespace Autocomplete dropdown during project creation.

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[illume]

Thanks!

Please consider the following commit message:

frontend: project: Disable assigned namespaces in project creation

Can you please check the open review comments?