Fix theme picker rendering the border colour as a fill - #624

The Theme picker drew as a grey block instead of a grid of cells.

ThemePickerRow uses a seam trick: a `gap-px` grid whose PARENT is painted with
the border colour, so only the 1px gaps show it. That depends on every cell
being opaque. The cells used `bg-row-bg` and `bg-row-selected-bg`, and both of
those tokens are deliberately see-through — `transparent`, and a 15% accent mix
against `transparent`. So the parent's border colour flooded every cell, and
the picker rendered with the border colour used as a fill.

Both are correct defaults for an ordinary row sitting on a panel; they are just
wrong under a parent painted with the border colour. `bg-row-hover-bg` was
always opaque (`--theme-surface-hi`), which is why hover was the one state that
looked right, and the selected cell only looked plausible because a 15% tint
over grey still reads as tinted.

Cells now name opaque tokens explicitly rather than depending on tokens whose
own definition documents them as transparent: `bg-panel-bg` for the resting
state, and a new `--theme-row-selected-solid-bg` that mixes the same 15% accent
against `--theme-surface` instead of `transparent`. Both derive from roles
Custom Appearance already exposes, so custom themes keep tuning it.

The new token is deliberately NOT added to the customAppearance role map: it is
a derived companion to an existing role, not a new role to configure.

ThemePickerRow is the only component using this seam pattern, so the blast
radius is one file plus the token.

Verified: tsc clean on both projects, 1721 tests / 251 files green.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>