* feat: add HEIC/HEIF image format support
Add detection, MIME type mapping, and dimension parsing for HEIC/HEIF
images via ISOBMFF ftyp brand inspection and ispe box parsing. Update
Gemini relay to accept these formats and refactor getImageConfig to
properly retry decoders using buffered data.
* fix: handle ISOBMFF extended sizes in HEIF dimension parser
parseHEIFDimensions now correctly handles boxSize==1 (64-bit extended
size) and boxSize==0 (box-to-EOF), preventing the parser from breaking
out of the loop when encountering these valid ISOBMFF box headers
before reaching the meta box.
