rt-client: bump to 0.31.0 (mattstack.mode retired)

[m4ttheweric]

• bumps @mattstack/rt-client to 0.31.0 so flavor: retire dev mode; the launcher sets the flavor and opening an app switches #418's registry change (mattstack.mode retired, isRetiredKey) can be published; the console still lists the key from 0.30.0
• unsetSetting removes a retired key a store still carries (it refused it as unknown, so rt settings unset mattstack.mode had no supported path)

2 new tests in write.test.ts; 51/51, tsc clean.

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