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feat(playground): show per-message cost using models.dev pricing

Adds a per-message USD cost next to the existing token breakdown on
assistant messages. Cost is computed from models.dev pricing
(USD/1M tokens), fetched once and cached on disk for 24h in the main
process. Cached input tokens use the model's cache_read rate when
available. Providers/models with no pricing entry (local providers,
unknown models) render exactly as before.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>