🤖 fix: correct token totals + refactor stats computations

- Don’t fold reasoning tokens into output token totals
- Extract StatsTab timing/model breakdown calculations into a util
- Share live timer via useLiveTimer + pass now into StatsTab
- Simplify sidebar cache equality via shallowEqual
- Add unit tests for timing stats (TTFT/tool time/persistence)

Change-Id: I548b1852f2d3a9b591ce7c70bd7b6c0e17f4b336
Signed-off-by: Thomas Kosiewski <[email protected]>

Author: ThomasK33