fix: preserve right panel open/close state when switching chats

[excpt0]

Summary

• Fix right panel (Tasks/Files) ignoring user's open/close preference when switching between chats
• Panel previously force-reopened on every chat switch due to two redundant setPanelOpen(true) calls and a route-change effect that always set panelOpen = isChatDetailRoute
• Extract panel state decision logic into a pure function (computePanelOpenOnRouteChange) with full unit test coverage

Root Cause

Two locations unconditionally forced the panel open:

1. AppShell.tsx route-change effect: setPanelOpenRaw(isChatDetailRoute) — always true for /chat/[id]
2. chat/[id]/page.tsx and SplitColumn.tsx: setPanelOpen(true) on every session load

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