fix: Server->Client Sync + File Locking repariert

Server->Client Sync:
- Server sendet Timestamps ohne Timezone (2026-04-11T12:49:24.735436)
- parse_from_rfc3339 braucht Timezone -> schlug still fehl
- Client dachte IMMER er sei neuer -> Upload statt Download
- Fix: parse_server_time() akzeptiert beides (mit/ohne Timezone)
- Probiert RFC3339, dann NaiveDateTime mit Microseconds, dann ohne

File Locking:
- open_cloud_file nutzte API-Clone vom SyncEngine (evtl. alter Token)
- Jetzt direkt state.api (immer aktueller Token nach Refresh)
- Lock wird zuverlaessig gesetzt beim Oeffnen von .cloud Dateien

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

clients/desktop/src-tauri/src/sync/engine.rs

@@ -109,8 +109,7 @@ impl SyncEngine {
                         let local_modified = std::fs::metadata(&local_path)
                             .and_then(|m| m.modified()).ok();
                         let server_modified = entry.updated_at.as_deref()
-                            .and_then(|s| chrono::DateTime::parse_from_rfc3339(s).ok())
-                            .map(|dt| std::time::SystemTime::from(dt));
+                            .and_then(parse_server_time);
 
                         let server_is_newer = match (local_modified, server_modified) {
                             (Some(lt), Some(st)) => st > lt,
@@ -344,8 +343,7 @@ impl SyncEngine {
                         let local_modified = std::fs::metadata(&path)
                             .and_then(|m| m.modified()).ok();
                         let server_modified = se.updated_at.as_deref()
-                            .and_then(|s| chrono::DateTime::parse_from_rfc3339(s).ok())
-                            .map(|dt| std::time::SystemTime::from(dt));
+                            .and_then(parse_server_time);
 
                         let server_is_newer = match (local_modified, server_modified) {
                             (Some(lt), Some(st)) => st > lt,
@@ -448,6 +446,24 @@ fn find_subtree(tree: &[FileEntry], folder_id: i64) -> Option<Vec<FileEntry>> {
     None
 }
 
+/// Parse a server timestamp (may or may not have timezone)
+fn parse_server_time(s: &str) -> Option<std::time::SystemTime> {
+    // Try with timezone first (RFC3339)
+    if let Ok(dt) = chrono::DateTime::parse_from_rfc3339(s) {
+        return Some(std::time::SystemTime::from(dt));
+    }
+    // Try without timezone (naive, assume UTC)
+    if let Ok(dt) = chrono::NaiveDateTime::parse_from_str(s, "%Y-%m-%dT%H:%M:%S%.f") {
+        let utc = dt.and_utc();
+        return Some(std::time::SystemTime::from(utc));
+    }
+    if let Ok(dt) = chrono::NaiveDateTime::parse_from_str(s, "%Y-%m-%dT%H:%M:%S") {
+        let utc = dt.and_utc();
+        return Some(std::time::SystemTime::from(utc));
+    }
+    None
+}
+
 pub fn compute_file_hash(path: &Path) -> String {
     let data = match std::fs::read(path) {
         Ok(d) => d,