Improve token cost freshness

[Finesssee]

Summary

• track local token-cost freshness independently from provider quota age and expose the cost timestamp
• retry fast cost failures while preserving timeout suppression
• coalesce bounded models.dev refreshes for unknown OpenAI/Anthropic models, retain cached priceable entries, and rescan once after successful refresh

Validation

• cargo test --manifest-path rust/Cargo.toml (563 + 1 passed)
• cargo test --manifest-path apps/desktop-tauri/src-tauri/Cargo.toml (306 passed)
• both cargo fmt checks
• both cargo clippy --all-targets -- -D warnings
• npm run build and locale parity (572 keys)
• git diff origin/main...HEAD --check