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/Runner/Runner.entitlements

@@ -6,5 +6,9 @@
 	<array>
 		<string>group.dev.ayushya.noo</string>
 	</array>
+	<key>keychain-access-groups</key>
+	<array>
+		<string>$(AppIdentifierPrefix)dev.ayushya.noo.shared</string>
+	</array>
 </dict>
 </plist>