Align calendar events with the half-hour grid

[arapsum]

Summary

• render each hour as two 30-minute slots with a muted midpoint rule
• keep grid, event layout, scrolling, and drag previews on one 2 px/minute scale
• add interaction/layout regression coverage and document the new slot behavior

Validation

• cargo +stable clippy --workspace --locked --all-targets --all-features -- -D warnings -W clippy::pedantic -W clippy::nursery -W rust-2018-idioms
• cargo test --workspace --locked --all-targets --all-features
• cargo fmt --all -- --check
• git diff --check