Fix react(set-state-in-effect) lint warnings in ThreadPage and BotEditorPage

[regnull]

Goal

Fix the three react(set-state-in-effect) warnings reported by pnpm lint.

Context

Running cd frontend && pnpm lint reports:

src/pages/ThreadPage.tsx:52:5    react(set-state-in-effect)
src/pages/ThreadPage.tsx:74:7    react(set-state-in-effect)
src/pages/BotEditorPage.tsx:43:107 react(set-state-in-effect)

Calling setState synchronously inside useEffect triggers an extra render. The usual fix is to derive the value during render, initialize the state directly, or reset it by giving the component a key. See React's You Might Not Need an Effect.

Acceptance criteria

• pnpm lint no longer reports these three warnings
• No behavior change: switching threads or bots still resets the page state as it does today
• pnpm test and pnpm typecheck pass, with a test added if a reset behavior wasn't covered before