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.

relay/channel/gemini/relay-gemini.go

@@ -37,6 +37,8 @@ var geminiSupportedMimeTypes = map[string]bool{
 	"image/jpeg":      true,
 	"image/jpg":       true, // support old image/jpeg
 	"image/webp":      true,
+	"image/heic":      true,
+	"image/heif":      true,
 	"text/plain":      true,
 	"video/mov":       true,
 	"video/mpeg":      true,