feat: keyboard-navigable login handle suggestions

Arrow keys and Tab now move the selection through the typeahead
list while the handle TextField stays focused. Up-arrow off the
first row deselects and returns the cursor to the TextField; Tab
wraps from the last row back to the first. Return submits the
highlighted row (or the raw text if nothing is selected) with the
same code path as a click.

The highlight itself is a single matched-geometry liquid-glass
layer tinted with the accent color, so moving between rows is a
physical slide rather than a color snap. The layer is inset from
the container edge and rounded, fixing the hard rectangular edges
that were bleeding into the surrounding glass card.

Also adds a ScrollViewReader `scrollTo` on highlight change so a
highlighted row that's outside the viewport (e.g. when the
keyboard has pushed the list) smoothly centers itself.