Add progress bars, fix DCT decode, sparkly MOTD

Progress bar support (v4.1.2):
- Web frontend: Real-time progress during encode with phase display
- CLI: --progress flag with rich library for encode command
- Backend: progress_file parameter for async progress reporting

DCT decode bug fix:
- Fixed InvalidMagicBytesError not being caught in early-exit check
- RS-protected format (v4.1.0+) has length prefix first, not magic bytes
- Exception handler now catches both ValueError and InvalidMagicBytesError

MOTD update:
- Added sparkly header to setup.sh MOTD (matches other rpi scripts)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontends/web/stego_worker.py

@@ -111,6 +111,7 @@ def encode_operation(params: dict) -> dict:
         dct_output_format=params.get("dct_output_format", "png"),
         dct_color_mode=params.get("dct_color_mode", "color"),
         channel_key=resolved_channel_key,  # v4.0.0
+        progress_file=params.get("progress_file"),  # v4.1.2
     )
 
     # Build stats dict if available