feat: add configurable Pomodoro duration

[okaryo]

Summary

• Add a Pomodoro timer duration setting
• Allow focus duration to be set from 1 to 60 minutes
• Persist the duration in local settings
• Update the Settings dialog UI for the Pomodoro controls

Checks

• pnpm run format:check
• pnpm run lint
• pnpm run check
• pnpm test
• pnpm run build
• cargo fmt --manifest-path src-tauri/Cargo.toml -- --check
• cargo clippy --manifest-path src-tauri/Cargo.toml --locked -- -D warnings
• cargo test --manifest-path src-tauri/Cargo.toml --locked

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve paused session when applying duration updates

setPomodoroDuration resets any non-running state to a fresh timer, which discards elapsed time for paused sessions. In the current flow (PomodoroSection calls this when settings change), if a user changes duration while a session is running and then pauses, the next state update resets remaining time to the full new duration even though they did not press Reset. This breaks expected pause/continue behavior and can silently lose in-progress focus time; the reset should be limited to truly idle states (e.g., not running and at full remaining time).

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