Product pivot: defer Home Assistant; provider-based server architecture (Nextcloud default)

ISOLATE (nothing deleted):
- moved HA module (ha/, ui/devices/, HA client tests), e2e scripts, and HA
  docs under deferred/home-assistant/ with a README explaining status + how
  to revive; complete snapshot preserved on branch deferred/home-assistant

REMOVE FROM ACTIVE PRODUCT:
- HomeScreen: Devices card + route gone; MainActivity nav updated
- ShonarApplication: haRepository removed
- BuiltInSettings: Home Assistant category/settings removed from defaults
- SettingsManagerTest: secret tests rewritten around a user-created
  SECRET-type setting (no built-in secret ships)
- Manifest + URL-validation test fixture wording neutralized
- README/ROADMAP: HA marked deferred with pointer to preserved branch

ADD (design, per product direction):
- docs/server-providers.md: ShonarProvider interface, Room data model,
  auth ladder (OIDC/PKCE -> Nextcloud login-flow-v2 -> token paste),
  sync strategy, provider-selection UX (Nextcloud default; Start9/Umbrel
  as platform-probe + explicit service binding, never universal APIs;
  custom SHONAR server; local-only), TLS TOFU pinning policy,
  no-secret-logging rules, provider contract test strategy, phased plan P0-P7

VERIFY: 19 Android unit tests green, APK builds, on-device launch OK
(consent dialog renders; no Devices entry). Backend unchanged (26 tests).

README.md

@@ -64,11 +64,16 @@ buildable. See [docs/ROADMAP.md](docs/ROADMAP.md) for the maintained matrix.
   limiting, chunked resumable uploads, recordings CRUD, storage
   abstraction, Postgres FTS schema, Docker dev stack.
 - Android: app shell with a generic data-driven **Custom Settings** system
-  (8 value types, add/edit/delete/reset/search/export/import), and a full
-  **Home Assistant** integration — connect, test, entity discovery, live
-  WebSocket state updates, service calls (toggles), encrypted token
-  storage. See [docs/home-assistant.md](docs/home-assistant.md).
-- Recording engine, SHONAR sync, playback, AI pipeline: TODO per roadmap.
+  (8 value types, add/edit/delete/reset/search/export/import).
+- **Product direction:** the server layer is provider-based, with
+  **Nextcloud as the default** provider and options for Start9, Umbrel, a
+  custom SHONAR server, or local-only storage. See
+  [docs/server-providers.md](docs/server-providers.md) for the interface,
+  data model, auth flow, and phased plan.
+- Home Assistant is **not** part of the initial product. Prior work is
+  preserved but disabled under [`deferred/home-assistant/`](deferred/home-assistant/README.md)
+  and on branch `deferred/home-assistant`.
+- Recording engine, provider sync, playback, AI pipeline: TODO per roadmap.
 
 ## AI providers