Feature/matugen on startup toggle

[njfletcher215]

Description

An option (Under Personalization > Theme & Colors > (NEW) Startup Behavior) to allow the user to choose between setting the theme on startup and waiting for an explicit change to the theme.

Label: Generate Theme at Startup
Type: toggle (boolean)
Default state: true (matches current behavior).

This allows users who have heavy matugen hooks to opt out of regenerating their theme every time dms starts, instead only regenerating when the wallpaper actually changes.

Note that the setting was placed immediately after the theme type selection, because it is not appliccable (and thus disabled) for a custom theme ("Custom" or "Browse" selected).

I also deduplicated some code in Theme.qml:Component.onCompleted (immediately after the guard that was inserted to enforce the setting).

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Refactor / internal cleanup
• Documentation
• Other

Related issues

Closes #3257

Screenshots / video

Checklist

• My code follows the conventions in CONTRIBUTING.md
• I have tested my changes locally
• New user-facing strings are wrapped in I18n.tr() with translator context, reusing existing terms where possible
• Go changes: ran make fmt, added/updated tests, make test passes, and go mod tidy is clean
• QML changes: ran make lint-qml with no new warnings
• I have opened a corresponding pull request in dlx-docs to document any new behaviors: https://github.com/AvengeMedia/DankLinux-Docs