Add PowerShell Last.fm OAuth setup script for Windows

[demoded]

PR Description

Summary

Adds a native Windows PowerShell script (setup-lastfm.ps1) mirroring the functionality of setup-lastfm.sh. This allows Windows users to complete the Last.fm OAuth authorization flow and configure credentials without needing Bash, curl, jq, or WSL.

Changes & Features

• Zero External Dependencies: Implements API signing and requests using built-in .NET cryptography (MD5) and Invoke-RestMethod — no external CLI utilities required.
• Windows Path & Config Detection: Automatically detects the Windows configuration path (%APPDATA%\cliamp\config.toml) while falling back to $HOME\.config\cliamp\config.toml. Also supports a -ConfigFile parameter for custom locations.
• Safe TOML Handling: Adds or updates credentials specifically within the [plugins.cliamp-lastfm] section without disturbing other plugin configs or file formatting.
• Enforced HTTPS: Uses HTTPS endpoints for all Last.fm API requests and authentication redirects.
• Browser Automation: Automatically opens the user's default browser to create API credentials and authorize the application session.

Verification

• Tested the step-by-step interactive flow on Windows.
• Verified token retrieval, browser authorization callback, session key exchange, and writing to config.toml.
• Confirmed successful scrobble