fix(security): add file content validation for map transfer

[bobtista]

• Closes Map transfers lack file content validation #1817

Implements file content validation during map transfer operations to prevent malformed or malicious files from being processed.

Implementation

Adds FileSystem::hasValidTransferFileContent() which validates transferred files after they are written to disk but before the transfer is marked complete. If validation fails, the file is automatically deleted.

Validation methods:

• .map files - Validates magic bytes CkMp (Chunky Map header)
• .ini files - Checks for null bytes to ensure text format (rejects binary)
• .tga files - Validates minimum size (18 bytes for valid TGA header)
• .str, .txt, .wak - Size validation only

Maximum file sizes:

• .map: 50 MB
• .ini: 10 MB
• .str/.txt: 5 MB
• .tga: 20 MB
• .wak: 10 MB