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/.kotlin/errors/errors-1788909278662.log

@@ -0,0 +1,4 @@
+kotlin version: 2.0.21
+error message: The daemon has terminated unexpectedly on startup attempt #1 with error code: 0. The daemon process output:
+    1. Kotlin compile daemon is ready
+