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).

android/app/build.gradle.kts

@@ -57,10 +57,10 @@ dependencies {
 
     // persistence
     implementation("androidx.datastore:datastore-preferences:1.1.1")
-    // keystore-backed secrets (Home Assistant tokens etc.)
+    // keystore-backed secrets (server provider credentials)
     implementation("androidx.security:security-crypto:1.1.0-alpha06")
 
-    // networking (Home Assistant REST + WebSocket; later SHONAR API)
+    // networking (server provider APIs over HTTPS)
     implementation("com.squareup.okhttp3:okhttp:4.12.0")
     implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.7.3")
     implementation("org.jetbrains.kotlinx:kotlinx-coroutines-android:1.9.0")