enhance: extract reusable CopyButton control with copy feedback

[JC-Chung]

What

Extracts the "copy + show checkmark for 1s" logic into a reusable
CopyButton control (src/Views/CopyButton.cs), and uses it in:

• CommitBaseInfo (copy commit SHA)
• LauncherPage (copy notification message)

Why

Both places previously implemented very similar copy-feedback logic
(a DispatcherTimer + two icons toggled by an IsCopied flag)
separately. CopyButton consolidates this into one control driven by
a CopyText property, so no changes to data models are needed.

Note

This overlaps with #2666, which adds the same copy-feedback UX to
LauncherPage via Notification.IsCopied. Left a comment there to
discuss which approach to land on.

[love-linger]

CopyText is a per-instance-data. I suggest to use DirectProperty instead of StyledProperty for it.

[JC-Chung]

Pushed two commits addressing this:

1. fix: ... — OnClick was setting IsCopied = true even when CopyText was empty, so it now returns early in that case. Also removed the now-redundant if (_resetTimer != null) check in OnUnloaded, since _resetTimer is always initialized in OnLoaded by the time OnUnloaded runs (same pattern as CommitBaseInfo's _iconResetTimer, which never null-checks it either).
2. code_style: ... — switched CopyText to DirectProperty as suggested, since it's purely per-instance data with no styling/animation use case, matching the existing IsCopied property.