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/Shared/DavFolders.swift

@@ -0,0 +1,120 @@
+import Foundation
+
+/// A folder in the account's WebDAV tree. [path] is relative to the user's
+/// root with a leading slash ("/Documents/Tax Forms"); "/" is the root.
+struct DavFolder: Equatable {
+  let name: String
+  let path: String
+}
+
+/// Parses a WebDAV `PROPFIND` multistatus into folders. Pure (no
+/// networking), so it's unit-tested. Compiled into both targets.
+enum DavFolderParser {
+  private struct Response {
+    var href = ""
+    var isCollection = false
+  }
+
+  private final class Delegate: NSObject, XMLParserDelegate {
+    var responses: [Response] = []
+    private var current: Response?
+    private var text = ""
+
+    private func local(_ name: String) -> String {
+      name.split(separator: ":").last.map(String.init) ?? name
+    }
+
+    func parser(
+      _ parser: XMLParser, didStartElement elementName: String, namespaceURI: String?,
+      qualifiedName qName: String?, attributes attributeDict: [String: String] = [:]
+    ) {
+      switch local(elementName) {
+      case "response": current = Response()
+      case "href": text = ""
+      case "collection": current?.isCollection = true
+      default: break
+      }
+    }
+
+    func parser(_ parser: XMLParser, foundCharacters string: String) {
+      text += string
+    }
+
+    func parser(
+      _ parser: XMLParser, didEndElement elementName: String, namespaceURI: String?,
+      qualifiedName qName: String?
+    ) {
+      switch local(elementName) {
+      case "href": current?.href = text.trimmingCharacters(in: .whitespacesAndNewlines)
+      case "response":
+        if let current { responses.append(current) }
+        current = nil
+      default: break
+      }
+    }
+  }
+
+  /// "/nextcloud/remote.php/dav/files/alice/Tax%20Forms/" -> "/Tax Forms".
+  /// The server may live under a sub-path, so the files root is located by
+  /// its marker instead of assumed to start at the beginning.
+  static func relativePath(fromHref href: String) -> String? {
+    let decoded = href.removingPercentEncoding ?? href
+    let marker = "/remote.php/dav/files/"
+    guard let range = decoded.range(of: marker) else { return nil }
+    var parts = decoded[range.upperBound...].split(separator: "/", omittingEmptySubsequences: true)
+    guard !parts.isEmpty else { return nil }
+    parts.removeFirst()  // the username
+    return "/" + parts.joined(separator: "/")
+  }
+
+  /// Child folders of [currentPath], sorted by name. The requested folder
+  /// itself (which a Depth-1 PROPFIND also returns) and - unless
+  /// [includeHidden] - dot-folders are left out.
+  static func folders(
+    from xml: Data, excluding currentPath: String, includeHidden: Bool = false
+  ) -> [DavFolder] {
+    let delegate = Delegate()
+    let parser = XMLParser(data: xml)
+    parser.delegate = delegate
+    parser.parse()
+
+    let current = currentPath == "/" ? "/" : currentPath.replacingOccurrences(of: "/+$", with: "", options: .regularExpression)
+    return delegate.responses
+      .filter(\.isCollection)
+      .compactMap { response -> DavFolder? in
+        guard let path = relativePath(fromHref: response.href), path != current else { return nil }
+        let name = (path as NSString).lastPathComponent
+        guard !name.isEmpty, includeHidden || !name.hasPrefix(".") else { return nil }
+        return DavFolder(name: name, path: path)
+      }
+      .sorted { $0.name.localizedCaseInsensitiveCompare($1.name) == .orderedAscending }
+  }
+}
+
+/// Lists folders over WebDAV for the Share Extension's picker, in the
+/// foreground (the sheet is on screen while it runs).
+enum DavClient {
+  private static let body = """
+    <?xml version="1.0"?>
+    <d:propfind xmlns:d="DAV:"><d:prop><d:resourcetype/></d:prop></d:propfind>
+    """
+
+  static func listFolders(account: SharedAccount, path: String) async throws -> [DavFolder] {
+    guard let url = WebDAV.fileURL(serverUrl: account.serverUrl, username: account.username, remotePath: path)
+    else { throw TransferError(message: "Invalid server address.") }
+    var request = URLRequest(url: url)
+    request.httpMethod = "PROPFIND"
+    request.setValue("1", forHTTPHeaderField: "Depth")
+    request.setValue("application/xml", forHTTPHeaderField: "Content-Type")
+    request.setValue(account.authHeader, forHTTPHeaderField: "Authorization")
+    request.httpBody = body.data(using: .utf8)
+    request.timeoutInterval = 20
+
+    let (data, response) = try await URLSession.shared.data(for: request)
+    let status = (response as? HTTPURLResponse)?.statusCode ?? 0
+    guard status == 207 else {
+      throw TransferError(message: status == 401 ? "Signed out - open Noo to sign in again." : "Server returned \(status).")
+    }
+    return DavFolderParser.folders(from: data, excluding: path)
+  }
+}