docs: add archived project notice (#43)

Agent-Logs-Url: https://github.com/unliftedq/PocketDesktop/sessions/02da22b7-805f-477c-8b04-7a46b394e065

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: unliftedq <9988582+unliftedq@users.noreply.github.com>

Author: Copilot

README.md

@@ -1,7 +1,12 @@
 # Pocket Desktop
 
+> [!WARNING]
+> Pocket has shut down, so Pocket Desktop no longer works.
+> This repository is now archived and will not receive further updates.
+
 Third-party desktop application for https://getpocket.com.
 
-Install from **[Microsoft Store](https://www.microsoft.com/store/apps/9N36D06FRZ4G)**
+The Microsoft Store listing is kept here for historical reference only:
+**[Microsoft Store](https://www.microsoft.com/store/apps/9N36D06FRZ4G)**
 
 ![Screenshot](./docs/images/Screenshot.png)