fix: optimistic user message echo + tabs persistence

Bug 1 (sent message didn't appear until turn end): the transcript only folded
assistant AgentEvents, so the user's own message showed only after turn-sealed
resync. Add core/chunks appendUserMessage() (provisional user chunk, superseded
on history sync) and call it in chat send() — the message now renders instantly.

Bug 2 (tabs didn't persist on refresh): the app passed { storage: undefined }
to createLocalStore, which the adapter treats as a no-op store, so nothing was
saved. Default to globalThis.localStorage. Regression test exercises the
non-injected path.

Also updated app store tests for the echo (assistant-vs-user chunk filtering).
Verified: svelte-check 0/0, vitest 288 (stable x2), biome clean, build ok.

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