feat: Cost per session breakdown (not just totals)

[vivekchand]

Overview

Show per-session cost and token usage in the Sessions tab, not just aggregate totals.

What to show

• Cost column in sessions list: $0.043
• Token breakdown: input / output / cache tokens per session
• Session cost chart: bar chart of top 10 most expensive sessions
• Alert threshold: flag sessions > $X (user-configurable)

Why

From the community: 'A runaway loop looks normal on a weekly total but obvious when you see 200 API calls in 3 minutes. Look for per-session or per-run breakdowns.' This is the #1 actionable metric for cost control.

Implementation notes

• Cost data is already in session JSONL (total_cost, total_tokens fields)
• Add cost column to sessions table (hidden by default on mobile)
• Sort by cost option in session filter
• Multi-provider: Claude, Gemini, OpenAI — per model rate table