fix #221: scale image pastes, drop text box, fix raw view and size

- image pastes render the image scaled to fit the viewer box (aspect
  ratio preserved, max-height 70vh), no text/code box below it
- link pill moved under the image as a small inline chip
- /raw serves image attachment bytes as an image instead of empty text
- view details size reports the actual attachment file size

internal/api/attachments.go

@@ -91,6 +91,17 @@ func (l *limitReader) Read(p []byte) (int, error) {
 	return n, err
 }
 
+// isImageMime reports whether the sniffed mime is a raster image the viewer
+// can render inline (#221). SVG is excluded: it is forced to text/plain on
+// serving by the active-content rule and must never render as an image.
+func isImageMime(mime string) bool {
+	switch mime {
+	case "image/png", "image/jpeg", "image/gif", "image/webp":
+		return true
+	}
+	return false
+}
+
 // handleCreatePasteMultipart implements POST /api/pastes with
 // multipart/form-data (#38). Fields mirror the JSON create path; a 'file'
 // part makes the paste a file paste (1 file = 1 paste: if text content is