Rebrand as HealthChat and pin the account provider to healthchat.ch

Applies the HealthChat product identity through the fork seams upstream already
provides, so no upstream logic files are touched and merges stay clean.

- Variants/HealthChat/healthchat.yml holds the identity scalars (display name,
  bundle id ch.healthchat.element, app group, development team) and is included
  permanently from project.yml. APP_NAME stays ElementX so the module name and
  @testable import ElementX keep working.
- ElementX/Sources/AppHooks/HealthChat/ registers an AppSettings hook pinning
  accountProviders to healthchat.ch with other providers disabled, push through
  sygnal.healthchat.ch and a custom-scheme OIDC redirect; plus a Compound hook
  recolouring the accent token family to the HealthChat blue.
- Entitlements are edited directly in ElementX/SupportingFiles/target.yml rather
  than in the variant, because XcodeGen concatenates arrays on include-merge and
  a variant can override scalars but cannot remove array entries. Drops the
  Element Classic app group and keychain group, and narrows associated domains
  to app.healthchat.ch, keeping matrix.to for permalinks.
- Info.plist CFBundleName now follows APP_DISPLAY_NAME — it, not
  CFBundleDisplayName, is what the web authentication sign-in alert shows.
- App logo and app icon assets swapped in place; the onboarding welcome title
  moves to a new screen_onboarding_welcome_title_ios untranslated string.

ElementX/Sources/AppHooks/HealthChat/HealthChatAppHooks.swift

@@ -0,0 +1,21 @@
+//
+// Copyright 2026 Novaloop AG
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+/// Registers the HealthChat hooks.
+///
+/// `AppHooks` is a concrete type and `AppCoordinator` calls `setUp()` on it directly, so this method
+/// wins over the empty `AppHooksProtocol` default via static dispatch — mirroring how Element's own
+/// Enterprise fork provides its `setUp()`. This file is compiled only into the main app; the NSE and
+/// Share Extension keep the empty default (they never drive authentication).
+extension AppHooks {
+    func setUp() {
+        #if IS_MAIN_APP
+        registerAppSettingsHook(HealthChatAppSettingsHook())
+        registerCompoundHook(HealthChatCompoundHook())
+        #endif
+    }
+}