fix: prevent /tmp permission corruption breaking system updates (#209)

Issue: LEDMatrix was changing /tmp permissions from 1777 (drwxrwxrwt)
to 2775 (drwxrwsr-x), breaking apt update and other system tools.

Root cause: display_manager.py's _write_snapshot_if_due() called
ensure_directory_permissions() on /tmp when writing snapshots to
/tmp/led_matrix_preview.png. This removed the sticky bit and
world-writable permissions that /tmp requires.

Fix:
- Added PROTECTED_SYSTEM_DIRECTORIES safelist to permission_utils.py
  to prevent modifying permissions on /tmp and other system directories
- Added explicit check in display_manager.py to skip /tmp
- Defense-in-depth approach prevents similar issues in other code paths

The sticky bit (1xxx) is critical for /tmp - it prevents users from
deleting files they don't own. Without world-writable permissions,
regular users cannot create temp files.

Fixes #202

Co-authored-by: Chuck <chuck@example.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

src/common/permission_utils.py

@@ -13,27 +13,63 @@ from typing import Optional
 
 logger = logging.getLogger(__name__)
 
+# System directories that should never have their permissions modified
+# These directories have special system-level permissions that must be preserved
+PROTECTED_SYSTEM_DIRECTORIES = {
+    '/tmp',
+    '/var/tmp',
+    '/dev',
+    '/proc',
+    '/sys',
+    '/run',
+    '/var/run',
+    '/etc',
+    '/boot',
+    '/var',
+    '/usr',
+    '/lib',
+    '/lib64',
+    '/bin',
+    '/sbin',
+}
+
 
 def ensure_directory_permissions(path: Path, mode: int = 0o775) -> None:
     """
     Create directory and set permissions.
-    
+
     If the directory already exists and we cannot change its permissions,
     we check if it's usable (readable/writable). If so, we continue without
     raising an exception. This allows the system to work even when running
     as a non-root user who cannot change permissions on existing directories.
-    
+
+    Protected system directories (like /tmp, /etc, /var) are never modified
+    to prevent breaking system functionality.
+
     Args:
         path: Directory path to create/ensure
         mode: Permission mode (default: 0o775 for group-writable directories)
-    
+
     Raises:
         OSError: If directory creation fails or directory exists but is not usable
     """
     try:
+        # Never modify permissions on system directories
+        path_str = str(path.resolve() if path.is_absolute() else path)
+        if path_str in PROTECTED_SYSTEM_DIRECTORIES:
+            logger.debug(f"Skipping permission modification on protected system directory: {path_str}")
+            # Verify the directory is usable
+            if path.exists() and os.access(path, os.R_OK | os.W_OK):
+                return
+            elif path.exists():
+                logger.warning(f"Protected system directory {path_str} exists but is not writable")
+                return
+            else:
+                raise OSError(f"Protected system directory {path_str} does not exist")
+
         # Create directory if it doesn't exist
         path.mkdir(parents=True, exist_ok=True)
-        
+
         # Try to set permissions
         try:
             os.chmod(path, mode)