Share Extension: account picker first, hidden folders follow the app's filter + unlock

The app now publishes every uploadable account, the active one, the three
lock settings and each account's own hidden-files filter (ShareAccountSync).
The sheet lists accounts first when there are several, asks for unlock to use
a non-active account (login lock + account-switching lock), and shows hidden
folders per the account's app setting behind the hidden-files unlock.

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

ios/ShareExtension/DeviceAuth.swift

@@ -0,0 +1,19 @@
+import LocalAuthentication
+
+/// Face ID / Touch ID with the device passcode as the fallback - the same
+/// policy the app's lock uses (`local_auth` with `biometricOnly: false`), so
+/// the share sheet asks for exactly what the app would.
+enum DeviceAuth {
+  /// True only if the user actually authenticated; a cancel, a failure, or a
+  /// device with nothing to authenticate with all count as "not unlocked".
+  static func authenticate(reason: String) async -> Bool {
+    let context = LAContext()
+    var error: NSError?
+    guard context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &error) else { return false }
+    return await withCheckedContinuation { continuation in
+      context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: reason) { success, _ in
+        continuation.resume(returning: success)
+      }
+    }
+  }
+}