refactor: fetch speed test servers on-demand instead of polling

[AustinChangLinksys]

Description

Remove GetCloseHealthCheckServers from core transaction to reduce polling payload. Server list is now fetched on-demand when entering the speed test view.

Changes:

• Remove server list fetch from polling core transaction
• updateServers() changed to fetchRemote: true for on-demand fetch
• Added loading spinner, error state with retry, and proper legacy fallback
• Differentiate between fetch error and empty server list

Reference

• PR: refactor: fetch speed test servers on-demand instead of polling #709
• Commit: e330989