Companion-app settings sync — register Android under Settings → Applications #76

What the web app has

/help/app-settings documents a whole subsystem: companion apps save their preferences to the
user's account so they travel between machines. Settings → Applications lists each app, the
machines signed in on it, and the settings stored — with a main workstation whose settings
seed any brand-new machine, plus rename, remove, view, copy to shared and delete shared.

Ten endpoints back it, none of them used by Android:

GET|PUT|DELETE /api/user/app-settings/{appKey} ·
GET /api/user/app-settings/{appKey}/bootstrap?deviceId= ·
GET|POST /api/user/app-settings/{appKey}/devices ·
PATCH|DELETE /api/user/app-settings/{appKey}/devices/{deviceId} ·
GET|PUT /api/user/app-settings/{appKey}/devices/{deviceId}/settings

Why it matters here

The Android app is a companion app. Registering it would mean a user's device-local
preferences (theme, feed defaults, the "add another after saving" toggle, sync cadence) follow
them to a new phone instead of resetting — and would make the app visible in the web's
Applications section alongside the Mac app.

Worth confirming first

Agree the appKey for the Android client with the owner before building anything.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.