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.

web/src/styles.css

@@ -101,10 +101,9 @@
     oklch(0.99 0.005 var(--preview-h)),
     oklch(0.19 0.045 var(--preview-h))
   );
-  --preview-tint: light-dark(
-    oklch(0.55 0.16 var(--preview-h) / 0.12),
-    oklch(0.78 0.14 var(--preview-h) / 0.16)
-  );
+  /* Opaque, not a wash: the previewed row reads as a fill, and a fill has to
+     land on the same colour whatever tier the row was already painted. */
+  --preview-tint: color-mix(in oklab, var(--preview) 30%, var(--surface-2));
   --preview-ring: light-dark(
     oklch(0.55 0.16 var(--preview-h) / 0.5),
     oklch(0.78 0.14 var(--preview-h) / 0.6)
@@ -1168,11 +1167,16 @@ textarea {
   background: color-mix(in oklab, var(--accent) 16%, var(--surface-2));
 }
 
-/* Where the keyboard walk (j/k) is standing. Distinct from `.is-active`: this
-   is a cursor, not a statement about what is mounted, so it reads as a ring
-   rather than a fill and the two can sit on the same row. */
+/* Where the keyboard walk (j/k) is standing. On a session row the cursor is
+   also what previews it, so it reads exactly like a hover — the preview hue,
+   deepening to the full fill once the preview lands. A folder row previews
+   nothing, so it keeps the plain cursor ring. */
 .tree-leaf.is-highlighted {
-  box-shadow: inset 0 0 0 2px var(--preview-ring);
+  background: color-mix(in oklab, var(--preview) 12%, var(--surface-2));
+}
+
+.tree-leaf.is-highlighted[data-status="running"] {
+  background: color-mix(in oklab, var(--preview) 18%, var(--surface-2));
 }
 
 .tree-folder-header.is-highlighted {
@@ -1198,9 +1202,10 @@ textarea {
    preview of an already-open session still reads as provisional. */
 .tree-leaf.is-previewing,
 .tree-leaf.is-previewing[data-status="running"],
-.tree-leaf.is-previewing.is-active {
+.tree-leaf.is-previewing.is-active,
+.tree-leaf.is-previewing.is-highlighted {
   background: var(--preview-tint);
-  box-shadow: inset 0 0 0 2px var(--preview-ring);
+  box-shadow: none;
 }
 
 /* Exited sessions recede: dimmed name + an "exited" badge carry status, so it's
@@ -3081,7 +3086,6 @@ input.leaf-rename:focus {
 .mv-pane.is-preview .attach-header,
 .mv-pane.is-preview.is-focused .attach-header {
   background: color-mix(in oklab, var(--preview) 34%, var(--surface));
-  box-shadow: inset 0 -1px 0 var(--preview-ring);
   opacity: 1;
 }
 /* A pane with no session yet: a quiet hint rather than blank terminal black. */