Centralize auto-update duration parsing

[ryanfowler]

Summary

• Added a shared duration parser in src/duration.rs and wired both config validation and auto-update execution to it.
• Extended auto-update intervals to accept fractional units, leading +, and d for day-sized intervals.
• Updated the configuration docs to match the supported syntax.
• Added tests covering 1.5h, +30m, 1d, and negative-duration rejection.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo test --all-features --test integration -- --test-threads=1