Fix Home Assistant instance credential updates

[SychPL]

Summary

• replace independent URL and token updates with a single connection update
• validate the candidate URL and token together in both manual and QR flows
• keep the stored connection unchanged when validation fails
• add regression tests for paired validation and persistence

Root cause

The settings screen validated a new URL with the stored token and a new token with the stored URL. When switching Home Assistant instances, neither mixed pair could authenticate, so users could not save either value.

User impact

Users can switch to another Home Assistant instance by submitting its URL and long-lived access token together.

Validation

• ./gradlew :app:testDebugUnitTest
• ./gradlew :app:assembleDebug