Concurrent failover switches for the same app could cause is_current,
local settings, and Live backup to point at different providers.
- Add SwitchLockManager with per-app mutexes (different apps still parallel)
- Unify scattered switch logic into ProxyService::hot_switch_provider
- Fix TOCTOU in set_current_provider via mutate_settings
- Add logical_target_changed to skip redundant UI refreshes
- Add tests for serialization and restore-waits-for-switch scenarios
Jason