Allow non-admin users to edit User Environment Variables and view System Environment Variables

### Description of the new feature / enhancement

Currently, PowerToys Environment Variables requires administrator privileges to fully work with environment variables.

It would be very useful if non-admin users could:

edit their own User Environment Variables
view System Environment Variables in read-only mode
use the same behavior as the native Windows Environment Variables dialog

The current Windows implementation already allows standard users to:

manage user-scoped variables without elevation
see system-scoped variables without editing permissions

PowerToys could follow the same permission model:

User variables → editable without admin rights
System variables → visible for all users, editable only with elevation

This would improve usability for developers and power users working in corporate or restricted environments where administrator permissions are limited.

### Scenario when this would be used?

Many developers work on company-managed devices without local administrator permissions.

Typical scenarios:

adding tools to PATH
configuring SDKs
setting application-specific environment variables
updating temporary development variables

Today, users often need to:

open the legacy Windows dialog
or ask administrators for unnecessary elevation

The native Windows tool already supports this workflow without requiring admin access for user variables.

PowerToys Environment Variables is much more modern and user-friendly, but currently loses this important capability.

This enhancement would allow PowerToys to become a full replacement for the native Windows environment variables editor.

### Supporting information

Suggested behavior:

Non-admin users can:
create/edit/delete User variables
view System variables
Admin elevation required only for:
creating/editing/deleting System variables

This matches existing Windows behavior and security expectations.

Possible UX idea:

System variables section visible but disabled/read-only when not elevated
"Run as administrator" button to enable editing system variables

The feature would especially help:

developers
CI/CD users
enterprise users
students on managed devices
users without permanent admin privileges

Start by locating the PowerToys Environment Variables utility and compare its current permission behavior with the native Windows Environment Variables dialog described in the issue. Define the user- and system-variable permission states, including read-only system variables and elevation for system edits; done means the proposed behavior works for both admin and non-admin users.

Written by the indexing model from the issue text.