fix(maestro): say preview with a fill, not a ring

The previewed row and the keyboard cursor both drew a 2px inset ring, which
read as a border on hover rather than as a colour. Rows now carry the preview
hue as a fill: a light wash under the pointer or the cursor, deepening to an
opaque `--preview-tint` once the preview lands.

That leaves `.is-active`'s ring as the only one in the session list, where it
now means exactly one thing — open in a pane. The pane's titlebar drops its
hairline for the same reason; the dashed edges that remain belong to the
canvas and the throwaway tab.

docs/PREVIEW.md

@@ -46,8 +46,12 @@ Web client (`--preview` and friends in `web/src/styles.css`):
 | The previewed pane's titlebar | Tinted in the preview hue, at full strength, in place of the focused pane's accent tint. |
 | The canvas | A `preview` chip, top right, and a dashed inset ring. |
 | A throwaway tab | Dashed border, italic label. |
-| The sidebar row being previewed | Preview tint + ring, over whatever open/focused state the row already had. |
-| A hovered sidebar row | Washed with the preview hue, because hovering is what previews it. |
+| The sidebar row being previewed | Filled with the preview hue, over whatever open/focused state the row already had. |
+| A hovered sidebar row, and the keyboard cursor on a session | A lighter wash of the same hue, because both are what previews a row. |
+
+Rows say it with fill, not with an edge: the only ring left in the session list
+is `.is-active`'s, which means "open in a pane". The dashed edges belong to the
+pane — the canvas ring and the throwaway tab.
 
 The hue is a palette token, not a fixed colour: a palette whose accent already
 sits on it moves it (`--preview-h`), so "previewing" and "picked" never collide.