refactor: swap device-id contexts for user-id contexts at call sites

Renames DecryptParams.ownDeviceId to ownUserId so decryptMessages actually
looks up bundles by user. Sweeps remaining OwnDeviceCtx and
loadDevicePrivateKey consumers in the desktop app to use cachedUserKey
from userIdentity. Files scheduled for deletion in later tasks
(BackupExportDialog, DeviceRestore, BackupRestoreDialog, BackupPromptBanner,
deviceBackup, DeviceRegistration) are left untouched.

packages/shared/src/chat/messages.ts

@@ -440,7 +440,7 @@ export async function removeReaction(
 export interface DecryptParams {
   client: AppSupabaseClient;
   messages: MessageWithCipher[];
-  ownDeviceId: string;
+  ownUserId: string;
   ownPrivateKey: Uint8Array;
   /**
    * Optional delegate that performs the symmetric-decrypt + utf-8 decode
@@ -483,7 +483,7 @@ export async function decryptMessages(opts: DecryptParams): Promise<DecryptedMes
       const handle = await tryGetConvKey(
         opts.client,
         m.conversationId,
-        opts.ownDeviceId,
+        opts.ownUserId,
         opts.ownPrivateKey,
         m.keyVersion,
       );