CI: mobile e2e smoke + drop SKIP_MOBILE from Makefile

[moodyjmz]

Follows Euro-Office/web-apps#157 / Euro-Office/web-apps#158 (merged), which removed SKIP_MOBILE so web-apps always builds and gates mobile. This is the DocumentServer half of the mobile-validation work originally scoped in Euro-Office/web-apps#154 (now closed — web-apps side is done). Context: the #258 regression shipped a broken mobile editor because nothing in CI loaded mobile.

Two items, both DocumentServer-side:

1. Mobile e2e smoke (the "does it actually load" gate)

web-apps now asserts mobile artifacts exist at build time (verify-deploy.mjs). That catches a missing/empty deploy — it does not catch a mobile editor that builds but renders a blank/404/"Not supported version" page. Add a Playwright test in e2e/ that:

• opens a mobile editor URL with a mobile UA / viewport,
• asserts the app shell mounts (not a 404 / error page).

Cost: ~one editor load. This is the deep gate that would have caught #258 at PR time.

2. Drop SKIP_MOBILE from the Makefile

develop/setup/Makefile still forwards SKIP_MOBILE to build-pipeline.js. Since Euro-Office/web-apps#158, the pipeline ignores it — so it's a silent no-op. Remove the param and any doc referencing it.

Acceptance

• A mobile editor that loads to a blank/404 page fails e2e.
• develop/setup/Makefile no longer references SKIP_MOBILE.