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>

rpi/first-boot-wizard.sh

@@ -390,7 +390,7 @@ gum style \
   --border-foreground 82 \
   --padding "0 2" \
   --align center \
-  "  ___  _____  ___    ___    _    ___    ___    ___" \
+  " ___  _____  ___    ___    _    ___    ___    ___" \
   " / __||_   _|| __|  / __|  /_\\  / __|  / _ \\  / _ \\" \
   " \\__ \\  | |  | _|  | (_ | / _ \\ \\__ \\ | (_) || (_) |" \
   " |___/  |_|  |___|  \\___//_/ \\_\\|___/  \\___/  \\___/" \