Unify Transmission settings requests to fix stale server updates and lost edits

[austin-smith]

Summary

• route session-get, session-set, free-space, port-test, and blocklist-update through TransmissionConnection and TransmissionStore
• replace cached settings request state with a shared SettingsViewModel bound to the active connection generation
• preserve newer edits when autosave overlaps with an in-flight session-set
• refresh the selected server immediately after host edits so updated endpoint and credentials take effect without reconnecting manually
• keep settings/session error presentation on the shared typed Transmission error mapper

This prepares the remaining Transmission write-side cleanup by removing the old settings/session request path.