fix MAM history loading, multi-device resource, and OMEMO self-session

Multi-device:
- Add DeviceResource with a stable per-install UUID-based resource
  (Luma-<hex>) persisted in UserDefaults and migrated from legacy
  "Luma".
- AccountConfiguration.effectiveResource falls back to
  DeviceResource.default
  for a blank or legacy resource; LoginView defaults resource to "" with
  an
  auto placeholder.
- Prevents two devices from sharing the same full JID resource and
  kicking
  each other off the stream (the "xmpp stream error" conflict).

MAM / history:
- loadOlderHistory now always issues the RSM page request instead of
  deferring, so scrolling up reliably loads older messages.
- Track interactive history mutations separately so live messages are
  not
  dropped while an older-history page is decoding.
- ChatView: replace the unreliable GeometryReader/preference scroll
  trigger
  with onAppear/onDisappear on a top sentinel; auto-load the first page
  when
  the conversation is empty.
- AppModel: reset hasMoreOlderHistory for empty conversations and clear
  isLoadingOlderHistory on disconnect.
- Decode own/duplicate/notEncrypted messages to
  decryption-failed/plaintext
  placeholders instead of dropping them, so every MAM stanza is
  collected.

OMEMO:
- Remove the encrypt-to-self setup that built a Signal session with the
  local
  device (cryptographically invalid), which caused an endless "Bad MAC"
  loop
  and a stream of undecryptable messages in the self-chat.
- Add LumaOMEMOStore.removeSessionWithOwnDevice() and call it on connect
  to
  purge any self-session persisted by the previous build.

Tests:
- Update AccountConfigurationTests for the unique resource and add
  coverage
  for legacy-resource migration and explicit-resource preservation.

Sources/Shared/Models/DeviceResource.swift

@@ -0,0 +1,30 @@
+import Foundation
+
+/// Stable, per-installation XMPP resource. Two devices that both fall back to
+/// the default must never share a resource: an XMPP server resolves a resource
+/// collision with a `conflict` stream error, which disconnects the older
+/// session. A UUID persisted in UserDefaults keeps the value stable across
+/// launches (so presence and stream-management resume stay consistent) while
+/// remaining distinct between macOS and iOS installs.
+enum DeviceResource {
+    /// The legacy hard-coded default that older Luma builds sent from every
+    /// device. Treated as "automatic" so existing accounts migrate without the
+    /// user having to re-enter credentials.
+    static let legacyDefault = "Luma"
+
+    private static let storageKey = "app.luma.xmpp.resource"
+
+    static var `default`: String {
+        let defaults = UserDefaults.standard
+        if let existing = defaults.string(forKey: storageKey), !existing.isEmpty {
+            return existing
+        }
+        let suffix = UUID().uuidString
+            .replacingOccurrences(of: "-", with: "")
+            .prefix(8)
+            .lowercased()
+        let resource = "Luma-\(suffix)"
+        defaults.set(resource, forKey: storageKey)
+        return resource
+    }
+}