Add iCloud attachment storage option

[visvig]

• Added a user setting to choose where attachment files live (on device vs iCloud Drive), stored in Defaults
• When iCloud is toggled and available, attachment paths (downloads/uploads/temp) point to the iCloud container; otherwise they stay in the app group
• Storage settings now show a picker for that choice and indicate when iCloud isn’t available
• Downloads first check for an iCloud copy and asks the system to fetch it; only hit Zotero/WebDAV if it’s missing
• Uploads can place the file into the same path under the iCloud container and still submit metadata to Zotero

Note:

• Built/Run in simulator
• iCloud path requires a real device with iCloud entitlements and iCloud Drive enabled (not validated here)