fix(connections): persist safe mode level as connection default

[J2TeamNNL]

Summary

• Safe mode changes in the toolbar now persist as the connection default across reconnects
• Previously only the in-session ConnectionSession.safeModeLevel was updated; ConnectionStorage was never written
• updateSafeModeLevel follows the save-before-markDirty ordering invariant; skips localOnly and sample connections

Test plan

• Change safe mode to Read-only, disconnect, reconnect → safe mode is still Read-only
• Change safe mode back to Off, disconnect, reconnect → safe mode is Off
• ConnectionStoragePersistenceTests.updateSafeModeLevelWritesThrough passes