Add Windows theme picker and scheduled rotation

[langyan1022]

This adds a small Windows theme picker and an optional scheduled rotation mode.

What changed:

• add a thumbnail-based saved theme picker for Windows
• apply a theme by selecting it in the picker
• mark the currently active theme in the picker
• allow deleting saved themes that are not currently active
• add optional theme rotation from the tray menu, with 15/30/60/120/240 minute intervals
• keep rotation off by default

The picker and rotation reuse the existing saved-theme flow, so theme validation and CSS rendering still go through the same paths as manual theme changes.

Checked locally with:

• Windows test suite: windows/tests/run-tests.ps1
• PowerShell parser checks for the updated Windows scripts