🤖 fix: prevent thinking block collapse on page load

[ethanndickson]

When navigating to a chat with existing thinking blocks, they would collapse immediately after the page loaded, causing a jarring layout shift.

Root cause: The ReasoningMessage component initialized isExpanded to true, then a useEffect collapsed it when isStreaming was false. For historical messages (already isStreaming: false), this effect fired on mount—expanding then immediately collapsing.

Fix: Initialize isExpanded to message.isStreaming. Historical messages start collapsed; actively streaming messages start expanded and collapse when streaming ends.

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