ux: show copy feedback icon when copying notification message

[weiningwei]

What

Shows a transient checkmark icon as visual feedback when the user copies a notification message, then resets back to the copy icon after one second.

Changes

• Make Notification observable (ObservableObject) and add an IsCopied property.
• Toggle between the copy icon and a green checkmark in the launcher notification item based on IsCopied.
• Add a DispatcherTimer in LauncherPage to reset IsCopied one second after copying.
• Add a tooltip to the copy button.

[JC-Chung]

Hi @weiningwei, thanks for this PR! I ran into the same need for CommitBaseInfo's SHA copy button and, as a follow-up, ended up building a reusable CopyButton control (#2668), covering both LauncherPage and CommitBaseInfo. It touches the same lines in LauncherPage.axaml(.cs), so merging both would conflict. Would you be open to rebasing this on top of CopyButton so we land on one consistent solution? Happy to help adjust either side.