fix: token usage reset handling in UI

[Dimillian]

Motivation

• Ensure the UI properly clears or updates thread token usage when the backend sends reset/null tokenUsage payloads so token usage bars reflect resets.

Description

• Allow thread/tokenUsage/updated handler to accept null payloads by updating the handler signature in src/features/app/hooks/useAppServerEvents.ts and changing the dispatch condition to allow explicit nulls.
• Accept null in the turn-events handler and pass it through by updating src/features/threads/hooks/useThreadTurnEvents.ts to accept tokenUsage: Record<string, unknown> | null.
• Normalize token usage inputs to safe default values by updating src/features/threads/utils/threadNormalize.ts so normalizeTokenUsage accepts null | undefined and uses an empty default object internally.

Testing

• No automated tests were run for this change.

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Codex Task

[Dimillian]

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Codex Review: Didn't find any major issues. 🚀

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