fix(page): page-host tag widget accepts comma-separated tags
Pasting or typing `book, fiction, novel` in the page-host floating
tag input previously created a single literal tag with the entire
string as its name (commas, spaces and all). The Enter handler now
splits on commas, trims each segment, drops empty segments, and
adds each as its own tag. Single-tag input is unchanged.
Tab-completion still operates on the whole input value — typing
`boo, fic` won't suggest tags starting with `fic`. Better
last-segment-aware completion is a separate scope.