iOS Share Extension: pick the destination inside the share sheet

SwiftUI folder picker in the extension (WebDAV PROPFIND), account shared via
a Keychain group, upload on a background session that outlives the extension
and is finished by the app. The inbox + notification hand-off remains as the
fallback (no account / 'choose later'). Shared code moves to ios/Shared.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ios/ShareExtension/Info.plist

@@ -20,6 +20,8 @@
 	<string>$(MARKETING_VERSION)</string>
 	<key>CFBundleVersion</key>
 	<string>$(CURRENT_PROJECT_VERSION)</string>
+	<key>NooKeychainAccessGroup</key>
+	<string>$(AppIdentifierPrefix)dev.ayushya.noo.shared</string>
 	<key>NSExtension</key>
 	<dict>
 		<key>NSExtensionAttributes</key>