feat: follow the pointer across displays

[CatStack-pixe]

Upstream source

• BongoCat PR #956 by @xuebkgithub
• Related report: BongoCat issue #895 by @imzry

Port assessment

Port as an opt-in multi-display behavior while retaining explicit window placement as the default.

Acceptance criteria

• The setting is persisted and can be disabled.
• Display changes do not produce duplicate windows or stale event listeners.
• Validate Windows, macOS, and Linux fallbacks separately.

[CatStack-pixe]

Closing as superseded/not planned. MochiPaw keeps explicit window placement as the default; multi-display dragging and physical monitor handling were implemented in PRs #22 and #39. The upstream pointer-follow behavior is outside the current product scope.