XAML: extract a shared dismissible-notification helper

Follow-up from the PR #50 review (finding L4).

Three notification modules implement the same "show a notification with a Don't show again action, persist the dismissal in globalState" pattern with near-identical code:

• src/xaml/csharpDevKitNotification.ts
• src/xaml/degradedNotification.ts
• src/xaml/projectRestoreNotification.ts

Each has its own state key handling, its own dismissal action wiring, and its own test file covering the same mechanics. A fourth such notification would mean a fourth copy.

Suggested work:

• Extract a shared helper (e.g. showDismissibleNotification({ key, message, actions, memento })) that owns the globalState read/write and the "Don't show again" action.
• Reduce the three modules to their message/action specifics.
• Collapse the duplicated portions of the corresponding tests.