fix(cloud-files): Sauberes Re-Register + FETCH_PLACEHOLDERS-Stub + mehr Log

- CfUnregisterSyncRoot VOR CfRegisterSyncRoot, damit alte Policies
  (z.B. PARTIAL) nicht durch UPDATE-Flag kleben bleiben
- FETCH_PLACEHOLDERS-Stub registriert, der mit leerer Antwort und
  DISABLE_ON_DEMAND_POPULATION-Flag antwortet. Safety-Net falls
  Windows trotz FULL-Policy doch danach fragt
- log_msg an kritischen Stellen (register, connect, populate), damit
  wir beim naechsten Timeout sehen wo es haengt

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clients/desktop/src-tauri/src/cloud_files/windows.rs

@@ -102,9 +102,18 @@ pub fn register_sync_root(
     // struct einbauen koennen. windows-rs verlangt hier nichts weiter.
     let _ = display_wide;
 
-    // Erst versuchen ohne UPDATE-Flag (frische Registrierung). Bei "schon
-    // registriert"-Fehler nochmal mit UPDATE-Flag. So laeuft es beim ersten
-    // Aktivieren UND bei Folgestarts.
+    // Erst eine eventuell vorhandene Registrierung wegraeumen. Sonst
+    // uebernimmt UPDATE nur einen Teil der Policies und alte PARTIAL-
+    // Population-Einstellungen bleiben aktiv -> Explorer-Timeout.
+    unsafe {
+        let _ = CF::CfUnregisterSyncRoot(PCWSTR(path_wide.as_ptr()));
+    }
+
+    log_msg(mount_point, &format!(
+        "register_sync_root path={} provider={} account={}",
+        mount_point.display(), provider_name, account_id
+    ));
+
     unsafe {
         if let Err(e) = CF::CfRegisterSyncRoot(
             PCWSTR(path_wide.as_ptr()),
@@ -112,9 +121,8 @@ pub fn register_sync_root(
             &policies,
             CF::CF_REGISTER_FLAG_NONE,
         ) {
-            let already = e.code().0 == 0x80070091u32 as i32  // DIR_NOT_EMPTY
-                || e.code().0 == 0x800710D1u32 as i32;         // Already registered
-            if already {
+            log_err(mount_point, &format!("CfRegisterSyncRoot FAILED: {e:?}"));
+            // Als Fallback mit UPDATE-Flag
             CF::CfRegisterSyncRoot(
                 PCWSTR(path_wide.as_ptr()),
                 &info,
@@ -122,13 +130,12 @@ pub fn register_sync_root(
                 CF::CF_REGISTER_FLAG_UPDATE,
             )
             .map_err(|e| format!("CfRegisterSyncRoot(UPDATE): {e}"))?;
-            } else {
-                return Err(format!("CfRegisterSyncRoot: {e}"));
-            }
         }
     }
 
+    log_msg(mount_point, "CfRegisterSyncRoot OK");
     connect_callbacks(mount_point)?;
+    log_msg(mount_point, "callbacks connected");
     Ok(())
 }
 
@@ -308,12 +315,40 @@ fn complete_transfer(
     Ok(())
 }
 
+unsafe extern "system" fn on_fetch_placeholders(
+    info: *const CF::CF_CALLBACK_INFO,
+    _params: *const CF::CF_CALLBACK_PARAMETERS,
+) {
+    // Safety-Net: wir populieren schon ueber populate_placeholders,
+    // aber falls Windows trotzdem ruft, geben wir leere Antwort.
+    let info = &*info;
+    let mut op_info = CF::CF_OPERATION_INFO::default();
+    op_info.StructSize = std::mem::size_of::<CF::CF_OPERATION_INFO>() as u32;
+    op_info.Type = CF::CF_OPERATION_TYPE_TRANSFER_PLACEHOLDERS;
+    op_info.ConnectionKey = info.ConnectionKey;
+    op_info.TransferKey = info.TransferKey;
+    let mut params = CF::CF_OPERATION_PARAMETERS::default();
+    params.ParamSize = std::mem::size_of::<CF::CF_OPERATION_PARAMETERS>() as u32;
+    let transfer = &mut params.Anonymous.TransferPlaceholders;
+    transfer.CompletionStatus = windows::Win32::Foundation::NTSTATUS(0);
+    transfer.PlaceholderTotalCount = 0;
+    transfer.PlaceholderArray = std::ptr::null_mut();
+    transfer.PlaceholderCount = 0;
+    transfer.EntriesProcessed = 0;
+    transfer.Flags = CF::CF_OPERATION_TRANSFER_PLACEHOLDERS_FLAG_DISABLE_ON_DEMAND_POPULATION;
+    let _ = CF::CfExecute(&op_info, &mut params);
+}
+
 fn connect_callbacks(mount_point: &Path) -> Result<(), String> {
     let callbacks = [
         CF::CF_CALLBACK_REGISTRATION {
             Type: CF::CF_CALLBACK_TYPE_FETCH_DATA,
             Callback: Some(on_fetch_data),
         },
+        CF::CF_CALLBACK_REGISTRATION {
+            Type: CF::CF_CALLBACK_TYPE_FETCH_PLACEHOLDERS,
+            Callback: Some(on_fetch_placeholders),
+        },
         // Sentinel: Type = INVALID beendet die Tabelle.
         CF::CF_CALLBACK_REGISTRATION {
             Type: CF::CF_CALLBACK_TYPE_NONE,
@@ -357,6 +392,9 @@ pub fn populate_placeholders(
     entries: &[RemoteEntry],
 ) -> Result<(), String> {
     use std::collections::HashMap;
+    log_msg(mount_point, &format!(
+        "populate_placeholders: {} Eintraege", entries.len()
+    ));
     let by_id: HashMap<i64, &RemoteEntry> = entries.iter().map(|e| (e.id, e)).collect();
 
     fn rel_path<'a>(