Single-file browser tool that decodes a base64 string and saves the
result back out as a file. No server, no dependencies, nothing uploaded.
Input cleanup covers data URIs, URL-safe alphabet, missing padding,
line wrapping, percent-encoding and wrapping quotes. Type detection
runs magic bytes (~30 signatures), then the data URI's declared MIME,
then UTF-8 content sniffing for SVG/HTML/XML/JSON/PEM; unrecognised
payloads fall back to .bin with a hex dump. Images, audio, video, PDF
and text are previewed inline before download.
