Field app: help-center "Mobile App" collection — install, sign-in, working offline, what "applied on sync" means, photos and walks on the phone

Goal

Customer documentation ships in the same PRs as the features (Help Center Documentation Gate), but the collection itself needs creating once: a "Mobile App" collection in Featurebase (mcp__featurebase__create_collection), then pnpm docs:sync:pull to materialise docs/help-center/mobile-app/collection.json (IDs never hand-written). Articles (via help-center-writer, IDs reserved with pnpm docs:sync:new): Installing the field app; Signing in and choosing your builder; Working offline and the sync indicator; "Applied when you're back online" (pay points, buyer photos); Lot walks on your phone; Photos on your phone; Troubleshooting sync (retry, discard, full resync). Existing articles that now describe the web only (Field View 1572819, Lot walkthroughs 3699906, Lot details 3797710) get a "On the mobile app…" section.

Acceptance criteria

• [ ] Collection exists and is committed; pnpm docs:sync and pnpm docs:audit pass (Overview → tasks → FAQ shape).
• [ ] Every in-app "Learn more" link in apps/mobile points at a reserved article id.
• [ ] Article URLs listed in this issue.