feat: show DeepSeek peak and off-peak pricing

[juanmackie]

Summary

• add the official DeepSeek peak/off-peak UTC schedule with the 2026-08-16 effective time
• show current period, local transition times, and effective time in DeepSeek cards
• poll status every minute and notify once when the observed pricing period changes
• add locale keys and deterministic schedule/UI tests

Verification

• cargo fmt --all -- --check
• cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
• cargo clippy --manifest-path apps/desktop-tauri/src-tauri/Cargo.toml --all-targets -- -D warnings
• cargo test --manifest-path rust/Cargo.toml (1209 passed)
• cargo test --manifest-path apps/desktop-tauri/src-tauri/Cargo.toml (341 passed)
• pnpm --dir apps/desktop-tauri exec vitest run (257 passed)
• pnpm --dir apps/desktop-tauri run build
• pnpm --dir apps/desktop-tauri run check-locale

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