Add workspace containers, environment management, and Postman variable import

[bajrangCoder]

Summary

This PR adds a complete environment-variable workflow and introduces real,
persisted workspace containers.

What changed

Environments

• Add named workspace and collection-scoped environments
• Resolve {{variable}} placeholders across URLs, headers, auth values,
  query parameters, and request bodies
• Support nested variables and detect missing or circular references
• Add enable/disable controls and masked secret values
• Add environment colors, duplication, rename, delete, and activation
• Add workspace/project precedence with project variables overriding workspace values
• Redesign the environment sidebar with:
  • active variable stack
  • grouped environment navigation
  • persistent variable editor
  • visible Add/New variable actions
  • Postman environment import action

Workspaces

• Add persisted workspace containers
• Add a top-toolbar workspace switcher
• Support creating, renaming, switching, and deleting workspaces
• Isolate collections, request history, and environments per workspace
• Migrate existing data into the default Personal Workspace
• Reset request tabs when switching workspaces to prevent cross-workspace references

Postman import

• Import standalone Postman environment JSON exports
• Import Postman collection variables
• Create a collection-scoped project environment for imported collection variables
• Preserve enabled/disabled and secret variable metadata
• Update import messaging and entry points for collections and environments

Data migration

Existing collections, history, and environments are automatically placed inside
the default Personal Workspace. No manual migration is required.

Todo

• Variable autocomplete, highlighting, and resolved-value previews