use_brand(): install and refresh the openwashdata brand

[larnsce]

Implements #109.

Summary

• use_brand() copies _brand.yml and the logo files it references from the central openwashdata/brand repository (raw GitHub at a configurable ref, or a local/alternative source for tests and offline work) into the active package.
• Refresh is idempotent: files compare by md5, unchanged files stay untouched, written and updated files are reported. Brand values are never edited locally (change-here-first discipline).
• With pkgdown = TRUE, an existing _pkgdown.yml is wired to the brand via template.bslib.brand; without a config the wiring is skipped with a setup_website() hint.
• brand.yml added to Suggests: bslib resolves the brand through it at site build time, surfaced by the end-to-end trial below.

Test plan

• Five new tests: install, idempotent refresh with an upstream change, pkgdown wiring and re-run stability, missing-config skip, missing-source error
• Full suite passes (63)
• End-to-end: use_brand(ref = "dev") on fslogisticskampala downloaded the brand over the network, wired _pkgdown.yml, and pkgdown::build_site() rendered the site in Atkinson Hyperlegible with the brand palette

Note: the default ref is main, which becomes functional once openwashdata/brand#1 merges; until then ref = "dev" exercises the real download path.