feat(app): add provider usage breakdown to session context

[Ayesy]

Summary

Add per-provider token usage breakdown to the session context usage tooltip, making it easier for users to understand their token consumption across different AI providers.

Changes

• Enhanced SessionContextUsage component with provider breakdown:

  • Shows per-provider token counts with emoji indicators (🟣 Anthropic, 🔵 Google, 🟢 OpenAI, etc.)
  • Displays percentage breakdown for each provider
  • Formats token counts (e.g., 1.2M, 500K)
  • Separates provider usage from context window stats

• Added SessionContextUsage to session header for better visibility

Screenshot

When hovering the usage indicator:

┌─────────────────────────────────┐
│ Provider Usage                   │
│ ─────────────────────────────── │
│ 🟣 Anthropic     1.2M      62%  │
│ 🔵 Google        500K      25%  │  
│ 🟢 OpenAI        250K      13%  │
│ ─────────────────────────────── │
│ Total            2.0M           │
├─────────────────────────────────┤
│ Context Tokens: 150,000         │
│ Window Usage: 45%               │
│ Total Cost: $2.50               │
└─────────────────────────────────┘

Why

Multi-model users often want to know which providers they're consuming the most tokens from. This helps with:

• Cost optimization (knowing which provider is being used most)
• Usage monitoring across different AI providers
• Better transparency in multi-agent workflows (e.g., oh-my-opencode)

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found a potentially related PR:

Related PR Found:

• PR feat: add /stats dialog with token usage statistics #10234 - feat: add /stats dialog with token usage statistics
  • feat: add /stats dialog with token usage statistics #10234
  • This PR addresses token usage statistics, which overlaps with your PR's focus on provider usage breakdown. It may have similar functionality or UI components that could be relevant to consider for consistency or potential consolidation.

This is worth reviewing to ensure there's no duplication of effort on token usage tracking and statistics display features.