[BUG]: App crashes when opening invalid RSS feed link

Bug Description
After modifying an existing RSS feed to use an invalid or malformed URL (e.g., a or bzsp://...), attempting to open the feed's link crashes the app with an ActivityNotFoundException.

To Reproduce
Steps to reproduce the behavior:

1. Long-press any existing feed to open the feed settings.
2. Long-press the feed link to open the edit dialog.
3. Change the link to an invalid URL (e.g., a or bzsp://...) and save.
4. Reopen the feed settings and click the modified link.
5. The app crashes.

Screen Recording
https://github.com/user-attachments/assets/0068f2a2-02df-4a75-a241-08ca62c8aa94

Expected Behavior
The app should validate the link format or handle the failure gracefully, showing an error or doing nothing instead of crashing.

Error Log

android.content.ActivityNotFoundException: No Activity found to handle Intent { act=android.intent.action.VIEW dat=bzsp://www.ruanyifeng.com//atom.xml }
    at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:2007)
    at android.app.Instrumentation.execStartActivity(Instrumentation.java:1673)
    ...
    at me.ash.reader.ui.ext.ContextExtKt.openURL(ContextExt.kt:485)
    ...

Smartphone:

• Device: generic_x86_64 (Android Studio emulator)
• OS: Android 9.0
• App version: 0.12.2

Start at ContextExt.kt:485, where the error log shows the app calls openURL, and trace the feed-settings link action that reaches it. Reproduce the invalid-link steps with values such as a or bzsp://..., then verify the app no longer crashes and instead validates the link or handles the failed intent gracefully.