fix(ui): whitespace-only and block-boundary selections no longer throw or create unanchorable annotations (#1584)

A double-click past the end of a block, or any selection whose text is only whitespace, used to throw an uncaught error from the highlighter or store a deletion whose quote was a bare newline, which on reload struck through every block below it. Blank or unserializable selections are now dropped before the containment check, the hook's own highlighter calls are guarded, and restore skips a blank quote. Normal selections produce byte-identical annotations. Closes #881.