Fix diary legacy storage and typed client data

[tiye]

Summary

• Normalize legacy storage with nil user/diary entries or nil diaries maps.
• Convert typed session messages to the plain maps expected by respo-message.
• Fix typed username lookup for login and sign-up.
• Convert a missing diary lookup from Option :none to nil before constructing ClientStore.

Verification

• Calcit 0.27.0: 8 tests passed; client and server entry checks passed.
• Existing storage copies decoded successfully; the account named chen is present in current data.
• Vite 8 production build passed with Node 24.19.0.

The running local native server must be restarted to load these changes. Isolated server execution was blocked by WebSocket FFI startup status 8, so the missing-diary path still needs a local UI check after restart.