fix(react-ui): don't auto-select the last tab on Tabs initial mount

[Shinyaigeek]

Fixes #819

Problem

<Tabs> selects the last tab on initial mount instead of the first, even for a fully static (non-streaming) response.

Standalone repro against the published packages: https://github.com/Shinyaigeek/openui-tabs-repro

The dependency-less auto-advance effect (streaming follow-along) runs its first pass with prevContentSizes.current empty, so every tab satisfies size > prevSize and candidate ends the loop at the last item. It runs in the same commit as — and after — the effect that defaults activeTab to the first tab, so the last tab wins.

Fix

Treat the first pass as a baseline-recording pass: record nextSizes but skip candidate selection when prevContentSizes.current is empty.

Streaming follow-along is unaffected:

• content growth relative to the recorded baseline still advances the active tab;
• a tab that appears after the first pass has no baseline entry, so its content counts as growth and still advances.

Verification

Rendered via a vite harness aliased to react-ui source, before/after:

Scenario	Before	After
Static 3-tab mount	last tab (Cherry) active	first tab (Apple) active
Streamed 3-tab response (12 chars / 150 ms)	follows growing tab, ends on Cherry	same — ends on Cherry

Before:

After (top: static mount now Apple; bottom: streaming control still ends on Cherry):

pnpm test (react-ui): 13 passed. lint:check: 0 errors. typecheck failures in UserMessageContent.tsx / index.ts are pre-existing on main (verified by stashing this change).

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