feat(dashboard): cost analytics panels — burn rate, token breakdown, cost by model

[OneStepAt4time]

Why

ccusage-dashboard (#3236) exposes 9 cost analytics panels. Aegis dashboard has zero cost visibility. Enterprise users need session burn rate, token breakdowns, and per-model cost tracking.

What

Design and implement 3 dashboard panels:

1. Session Burn Rate — cost over time chart (line chart, USD on Y axis, time on X). Shows spending velocity per session.
2. Token Breakdown — stacked bar chart: input tokens, output tokens, cache-read, cache-write per session. Toggle by time range.
3. Cost by Model — horizontal bar chart: total USD grouped by model (claude-3-opus, gpt-4o, gemini-pro, etc.).

Backend API (#3264) provides the data. This issue covers the frontend panels only — mock data is fine for initial implementation, wire to real API once #3264 ships.

Design references

• https://github.com/Nitjsefnie/ccusage-dashboard — 9-panel reference (study panel layouts)
• Existing Aegis dashboard patterns (use current chart library, Tailwind styling)

Done when

• 3 panels render with mock data
• Responsive layout (works on 1280px+ screens)
• Accessible (aria labels, keyboard nav, screen reader)
• Matches existing dashboard design language
• Loading/error/empty states handled

Priority: P2 — enterprise differentiator

Depends on: #3264 (cost tracking API — for real data)
Refs: #3236

[aegis-gh-agent]

Resolved by PR #3275 (merged on 2026-05-13).