Add a Script action type for running inline PowerShell

[danielchalmers]

Resolves #58.

Adds a new Script action type: write a multiline PowerShell script directly in the action editor and run it on click, with no separate .ps1 file:

• The script body is passed to the interpreter via -NoProfile -ExecutionPolicy Bypass -EncodedCommand <base64>, where the Base64 is the UTF-16LE bytes of the script. This means multiline bodies, quotes, and newlines need no escaping and no temp file (nothing to clean up).
• Runs hidden by default (UseShellExecute=false + CreateNoWindow). A fresh Script action defaults to hidden; once a script is written, the hidden choice is preserved across type toggles.
• Interpreter defaults to powershell.exe and is editable (e.g. pwsh.exe for PowerShell 7).
• Optional working directory. A blank script ("Script is empty") and an over-long script that would exceed the command-line limit ("Script is too long to run") surface clear messages through the existing tray notification path.

This differs from the Open type (which shell-executes a target and can't run a script silently): Script takes a script you write and runs it through PowerShell with a hidden console.

Notes

• Branched off main after Rename the Shell action type to Open #67 (the Shell → Open rename); Script is added alongside Open.
• Respects the existing invariant that actions never execute during tests, settings load, preview, or validation — only on a slice click.
• Script and RunHidden are new serialized PieAction fields; old settings files load fine (missing values default), and NormalizeAfterLoad handles the new Script field.