Separate context window and session stats in sidebar

[R44VC0RP]

Issue

On some /compact instances the session costs would be reset and the token count isn't ongoing which is nice for visuals.

Summary

• Split the sidebar Context section into two: Context (current window) and Session (cumulative)
• Context shows tokens and % used from the last assistant message (resets on compaction)
• Session shows total tokens across all messages and cumulative spend (only increases)
• Improves visibility into both current context usage and overall session costs

Changes

• Added sessionTokens memo that sums tokens from all assistant messages
• Moved spend to Session section alongside total tokens
• Kept Context section focused on current LLM context window