feat(app): Add OAuth rate limits and usage dashboard

[mguttmann]

Summary

Adds visibility into OAuth rate limits and usage information for authenticated providers, particularly useful for Claude Max users to monitor their 5-hour and 7-day usage limits.

Closes #8911

Changes

Backend (packages/opencode)

• src/auth/index.ts: Add getUsage() and fetchAnthropicUsage() functions to OAuthPool
• src/server/server.ts: Add GET /auth/usage API endpoint
• src/cli/cmd/auth.ts: Add opencode auth usage CLI command

Frontend (packages/app)

• src/components/dialog-auth-usage.tsx: New dialog component with:
  • Provider list with account counts
  • Anthropic usage limits visualization (5-hour, 7-day)
  • Color-coded progress bars (green/yellow/red)
  • Account status with cooldown indicators
  • Refresh button
• src/pages/layout.tsx: Add sidebar button to open usage dialog

Screenshots

The dialog displays:

• Usage bars showing utilization percentage
• Reset time countdown
• Active account indicator
• Cooldown status when rate-limited

Testing

• [x] Tested with Claude Max account
• [x] CLI command works correctly
• [x] API endpoint returns expected data

Note

This PR includes the OAuth multi-account failover commits from @gwizz as a foundation, since the usage dashboard builds on that infrastructure. Credit to gwizz for the excellent OAuth rotation system!