Add an optional Liquid Glass style for blur surfaces

[kiteretsu903]

Adds a Liquid Glass option for the snap preview, minimum-size warning, and stacked-window list on macOS 26 or later. These surfaces use Apple's native, untinted glass without the custom blur tint, border, or shadow.

When the user enables blur for the first time, Glass is selected. They can switch back to standard blur; older macOS versions keep the existing appearance.

This PR is independent of #1887 and contains only the glass setting, surfaces, and their regression tests. Synchronized preview geometry is handled in #1886 without adding a dependency to this PR.

Validation: all 6 Glass tests passed on this branch, covering cold/repeated animated previews, retargeting, material switching, closing, transparency, settings, and content preservation. Combined with #1886, real drag E2E and recording inspection also passed, with 0 pt sampled material size mismatch.

[kiteretsu903]

Closing in favor of the focused replacement: #1904.

The changes have been reorganized to keep each feature easier to review and limit changes to shared infrastructure. Prerequisite-dependent replacements are drafts and explain which earlier changes are included. The original discussion remains here for reference.