[FEATURE]: Performance tab with TTFT / tok/s / wall time per LLM call in the web app

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When a session feels slow there's no way to tell from the web app whether the time went to the provider (slow first token, slow generation) or to tool execution. The Context tab shows token usage but nothing about latency.

I'd like a Performance tab in the session side panel, opened from the command palette the same way Context is, that shows per LLM call:

• TTFT (time to first output)
• tokens/second during generation
• end-to-end wall time
• output tokens, model, and running/error state

plus session totals (median TTFT, tok/s, total wall, call count).

Most of this is already in the message data (time.created / time.streamed / time.completed, tokens, reasoning.time, tool.time). The one gap is that AssistantText has no timestamp, so a text-only reply has no first-output time. Adding AssistantText.time like reasoning and tool already have closes that.

I have a working implementation and will open a PR against v2.