Fix: Navigator Permissions - absolute Pfade für chmod/chown

- /bin/chmod statt chmod (line 435)
- /bin/chown statt chown (line 476)
- /bin/chmod -R statt chmod -R für recursive (line 514)

Behebt: 'Failed to save permissions' Fehler beim Ändern von Dateiberechtigungen

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/services/file_manager.py

@@ -432,7 +432,7 @@ class FileManager:
             mode_int = int(mode, 8)
 
             result = subprocess.run(
-                ["chmod", mode, str(target)],
+                ["/bin/chmod", mode, str(target)],
                 capture_output=True,
                 timeout=10
             )
@@ -473,7 +473,7 @@ class FileManager:
                 chown_spec = owner
 
             result = subprocess.run(
-                ["chown", chown_spec, str(target)],
+                ["/bin/chown", chown_spec, str(target)],
                 capture_output=True,
                 timeout=10
             )
@@ -511,7 +511,7 @@ class FileManager:
             import subprocess
 
             result = subprocess.run(
-                ["chmod", "-R", mode, str(target)],
+                ["/bin/chmod", "-R", mode, str(target)],
                 capture_output=True,
                 timeout=30
             )