Serve gzipped web UI assets

[Sgw32]

Motivation

• Serve pre-compressed frontend assets from flash with the correct HTTP headers so browsers accept gzip-encoded content.
• Ensure binary assets (PNG) remain unmodified while text assets are compressed and embedded for smaller firmware size.

Description

• Updated ESP32/Digifiz/main/digifiz_ws_server.c to serve the embedded gzipped HTML blob using the _gz_start/_gz_end symbols and to set httpd_resp_set_type(req, "text/html") plus the Content-Encoding: gzip header.
• Updated ESP32/Digifiz/main/CMakeLists.txt to find_program(gzip), generate a data/dist output, produce .gz outputs for non-PNG files, register those files via EMBED_FILES, add a frontend_assets custom target and add_dependencies(${COMPONENT_LIB} frontend_assets) so assets are built before the component.
• Added a small helper ESP32/Digifiz/main/compress_files.py to reproducibly create compressed artifacts offline and to copy PNGs unchanged.

Testing

• No automated tests or CI jobs were run for these changes.

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Codex Task