fix(layout): bandeau d'erreur cellule au-dessus des boutons de contrôle (#48)

Corrige la superposition z-index dans LeafView : le bandeau d'erreur de
cellule passait sous les boutons de contrôle, rendant le message illisible.
Le voile d'annonces ciblées est réaligné en conséquence.

Couvert par un nouveau test de layering (128/128 frontend au vert QA).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/layout/LayoutGrid.tsx

@@ -252,6 +252,28 @@ function goToCell(nodeId: string): void {
   }, 1200);
 }
 
+/**
+ * Local z-index scale for a single cell (ticket #48). Intra-cell layering only —
+ * unrelated to the global {@link zIndex} scale used for app-level surfaces.
+ *
+ * The cell controls (agent selector, split, close) must stay reachable even when
+ * a non-modal banner is shown, otherwise a cell stuck in an error state (e.g. an
+ * agent launch failure) can no longer be managed. Full-cell veils (write-portal
+ * injection, F3 busy) deliberately cover the terminal and sit above the banners,
+ * but they are `pointer-events:none` and stay *below* the controls so the user
+ * can always re-pick an agent / split / close the cell.
+ *
+ *   terminal          0   (implicit — the xterm surface)
+ *   status/error      3   (in-cell status strip, launch-error banner, notices)
+ *   full-cell veils   4   (write-portal + F3 overlays; mutually exclusive)
+ *   cell controls     5   (always on top and clickable)
+ */
+const CELL_Z = {
+  banner: 3,
+  veil: 4,
+  controls: 5,
+} as const;
+
 function LeafView({
   id,
   session,
@@ -597,7 +619,9 @@ function LeafView({
           // edge, ~15px wide). Without this offset the right-most control — the
           // close button — sits *behind* the scrollbar and is hard to click.
           right: 16,
-          zIndex: 2,
+          // Above every banner/veil so a cell in an error state stays manageable
+          // (ticket #48).
+          zIndex: CELL_Z.controls,
           display: "flex",
           gap: 2,
           alignItems: "center",
@@ -711,7 +735,7 @@ function LeafView({
             position: "absolute",
             top: 30,
             left: 4,
-            zIndex: 3,
+            zIndex: CELL_Z.banner,
             display: "flex",
             maxWidth: "calc(100% - 8px)",
             alignItems: "center",
@@ -878,7 +902,7 @@ function LeafView({
             style={{
               position: "absolute",
               inset: 0,
-              zIndex: 4,
+              zIndex: CELL_Z.veil,
               display: "flex",
               alignItems: "center",
               justifyContent: "center",
@@ -886,6 +910,7 @@ function LeafView({
               color: "var(--color-content, #e0e0e0)",
               fontSize: 13,
               pointerEvents: "none",
+              // veil tier — below the cell controls (ticket #48).
               userSelect: "none",
             }}
           >
@@ -924,7 +949,7 @@ function LeafView({
             background: "var(--color-surface, #1e1e1e)",
             padding: "2px 6px",
             borderRadius: 3,
-            zIndex: 3,
+            zIndex: CELL_Z.banner,
             display: "flex",
             alignItems: "center",
             justifyContent: "space-between",