fix(kit): merge initialValue defaults when syncing client shared state

[antfu]

Description

The client-side shared state currently replaces its state entirely with server values (state.mutate(() => serverState)), without merging the client's initialValue defaults. This means if the server state is missing a newly added field (e.g. commandShortcuts from stale persisted data), the client loses it — causing "Cannot set properties of undefined" errors.

This fix mirrors the server-side createStorage merge strategy ({ ...initialValue, ...savedValue }) on the client side, ensuring new default fields are always preserved when syncing with the server. This prevents the entire class of "new settings field is undefined" bugs.

closes #263

Linked Issues

closes #263
Alternative to #264

Additional context

As noted in #264, the proper fix is in the merge layer rather than adding per-field guards in UI components. The merge is applied in all three code paths where server state replaces client state: initial fetch, live update events, and the no-initialValue branch.

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commit: 586c6bb