fix: prevent nostr: prefix on bech32 IDs inside URLs and improve relay errors

[barrydeen]

Summary

• Fix bare bech32 auto-prefix regex incorrectly matching npub1/note1/nevent1/nprofile1 strings embedded in URLs (e.g. blossom band hostnames like npub1xxx.blossom.band), which caused published content to contain mangled URLs like https://nostr:npub1xxx.blossom.band/...
• Use exception class name instead of generic "Unknown error" for relay connection failures
• Suppress relay connection error console logs when app is backgrounded

Test plan

• Compose a note containing a blossom URL with an npub in the hostname and verify the URL is not modified on publish
• Paste a bare npub/note ID and verify it still gets auto-prefixed with nostr:
• Verify relay connection errors show the actual exception type instead of "Unknown error"
• Background the app and verify relay errors don't flood the console log