[OCaml] Mobile-friendly clone of cgit.
refactor name the compact nav by form, not by device
site_nav and repo_nav name the scope their links cover; mobile_nav named a device class instead, breaking the family's axis. It is repository-scoped like repo_nav, differing only in being condensed and pinned to the bottom, and nothing in the module knows what hardware is in use — CSS reveals it by viewport width. Rename it compact_repo_nav, which places it on the same axis as its siblings and states its relationship to repo_nav. Layout's bindings follow suit: mobile becomes compact_nav, and header_node becomes banner to match Ui.page_banner. This also changes the landmark's accessible name from "Mobile navigation" to "Compact navigation", the one rendered-HTML change in the series. The old label told screen-reader users something untrue, since the nav depends on viewport width rather than device. Verified as the only markup difference: 26 changed lines across 13 pages, all this attribute.