fix(init): use System PATH on Windows for correct priority

[CalvinAllen]

Description

On Windows, the dtvem init command adds the shims directory to the User PATH. However, Windows combines PATH as System PATH + User PATH, meaning system installations (like C:\Program Files\nodejs) always take priority over dtvem shims.

This causes dtvem-managed versions to never be used when a system-level installation exists.

Root Cause

• Windows PATH order: System PATH comes before User PATH
• PowerShell 7 also prepends $PSHOME and reconstructs PATH with System before User
• Adding to User PATH will never take priority over System PATH entries

Solution

• Add shims to System PATH instead of User PATH
• This requires administrator privileges
• Prompt user for UAC elevation when needed
• Re-launch dtvem init with elevation to make the change

Acceptance Criteria

• dtvem init modifies System PATH on Windows (not User PATH)
• Prompts for admin elevation if not already elevated
• Uses ShellExecute with "runas" verb for UAC prompt
• Works correctly when already elevated
• Unix/macOS behavior unchanged (prepend to shell config)