feat(browse): make virtual tree rows visually distinct

Folders the cascade declares but disk doesn't carry (working/,
staging/, reviewing/, mdl/, the canonical folders before they're
materialised) previously got just opacity:0.65 + an "(empty)" hint —
easy to miss, especially next to dimmed-but-real items.

Now they read as placeholders at a glance:

- Dashed left rail (2px, accent-muted) inside the row gutter.
- Italic label in muted text color.
- Lucide icon switches to outline-only (fill:none + stroke:currentColor)
  so virtual folders look sketched, not filled.
- "(empty)" hint italic + accent-muted to match the rail.
- Selected virtual row keeps the rail but switches it to the
  selection accent so "selected + placeholder" reads as both.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

browse/css/tree.css

@@ -496,19 +496,52 @@ body {
     word-break: break-all;
 }
 
-/* Virtual rows: synthesized client-side for folders that aren't on
+/* Virtual rows: synthesized for folders/files declared by the
-   disk yet (canonical project folders). Rendered muted so the user
+   cascade but absent from disk. The visual language reads as
-   reads them as "available but empty" rather than ordinary entries.
+   "expected, not yet materialized" — italic label, muted accent
-   Hover/select states still apply; the hint sits to the right of the
+   color, dashed left rail, and an outlined icon. Hover/select
-   label. */
+   chrome still applies on top; the dashed rail sits inside the row
-.tree-row--virtual .tree-name__icon,
+   so it doesn't fight padding-left indentation. */
-.tree-row--virtual .tree-name__label {
+.tree-row--virtual {
-    opacity: 0.65;
+    box-shadow: inset 2px 0 0 0 transparent;
+    position: relative;
 }
+.tree-row--virtual::before {
+    content: '';
+    position: absolute;
+    top: 4px;
+    bottom: 4px;
+    left: 2px;
+    border-left: 2px dashed var(--accent-muted, #8aa4cc);
+    pointer-events: none;
+}
+.tree-row--virtual .tree-name__label {
+    font-style: italic;
+    color: var(--text-muted, #6b7280);
+}
+.tree-row--virtual .tree-name__icon {
+    /* Hollow out the filled Lucide glyph: reduce fill opacity so
+       the icon reads as an outline-only sketch — the conventional
+       "placeholder, not actual" cue across UI systems. */
+    opacity: 0.5;
+}
+.tree-row--virtual .tree-name__icon svg {
+    fill: none;
+    stroke: currentColor;
+    stroke-width: 1.5;
+    stroke-linecap: round;
+    stroke-linejoin: round;
+}
+.tree-row--virtual.is-selected::before {
+    /* Selected virtual row: rail brightens to selection accent so the
+       row reads as both selected and placeholder. */
+    border-left-color: var(--accent, #2868c8);
+}
+
 .tree-name__hint {
     margin-left: 0.5rem;
     font-size: 0.78rem;
-    color: var(--text-muted);
+    color: var(--accent-muted, #8aa4cc);
     font-style: italic;
 }