perf(chat): cut per-message re-renders and O(window) work in the open chat

[f-liva]

Summary

On a web tab that stays open for hours on a chat with thousands of messages, every message applied to the open session re-rendered far more than the row it touched. SessionViewLoaded keyed its usageData memo on the session object identity, which applyMessages mints fresh on every batch once the session has usage, so AgentInput and everything below it re-rendered per message (sources/-session/SessionView.tsx). ChatListInternal and NewerEnd subscribed to the whole session object through useSession while reading three booleans, so activeAt heartbeats, usage updates and the 2 s draft saves while typing re-ran the whole list pipeline and FlashList (sources/components/ChatList.tsx). After scrolling to the top once, oldestRenderedId never moved back, so windowing, grouping, copy-text and list building ran over the entire history on every later update. buildAgentTurnCopyTextByMessageId re-joined the text of every completed turn on each of those runs (sources/utils/agentTurnCopy.ts), and FlashList received a fresh contentContainerStyle, onLayout, header and footer on every render.

This PR keys the usage memo on primitives, subscribes the list to exactly the three flags it reads (useShallow), shrinks the render window back to INITIAL_WINDOW when the reader is back at the newest message, caches each turn's copy text by final message id (bounded, wiped at 2000 entries), memoizes the FlashList props on their real inputs, and drops a per-render console.log in ToolView that fired for every errored tool row.

Changes

• Key SessionViewLoaded's usageData memo on the usage fields and subscribe to hasMessages/isLoaded booleans instead of the messages array
• Subscribe ChatListInternal and NewerEnd to thinking, pending-permission and controlledByUser via a useShallow selector instead of the whole session
• Reset the render window to INITIAL_WINDOW in scrollToBottom and when a scroll event lands on the newest message
• Cache buildAgentTurnCopyTextByMessageId output per final message id while the turn's message identities are unchanged
• Memoize FlashList contentContainerStyle, onLayout, ListHeaderComponent and ListFooterComponent
• Drop the per-render console.log of the tool result for errored tool rows in ToolView

Test plan

• pnpm typecheck (clean)
• pnpm exec vitest run sources/components/ChatList.test.ts sources/utils/agentTurnCopy.test.ts sources/hooks/useGroupedMessages.test.ts (25 passed; agentTurnCopy.test.ts "reuses the joined text of a turn whose messages are unchanged" fails without the cache)
• Ran on a self-hosted web deployment: scrolled a multi-thousand-message chat to the top and back to the bottom without jumps or reflow; window shrank back once at the newest message; typing in the composer and incoming messages no longer re-render the list; copy-turn text unchanged

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