feat(kanban): sidebar edge zone now toggles (open + close)

The 32px left drag zone now also closes the sidebar when it is open.
Symmetric behaviour: dwell ≥400ms while dragging closes if open / opens
if closed.

To prevent a drag that starts with the pointer already inside the
sidebar (e.g. dragging a sidebar card itself) from immediately auto-
closing, the close action requires the pointer to have left the strip
at least once after the drag started. So:
- closed + drag-to-edge -> opens (unchanged).
- open + drag a card out, then move the card back to the edge -> closes.
- open + drag a sidebar card directly to the board -> nothing happens.

After a successful toggle the dwell flag resets, so the user must leave
the strip and re-enter to trigger another action.

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

frontend/src/App.tsx

@@ -193,6 +193,12 @@ export function App() {
     }
     let timer: number | null = null;
     let inside = false;
+    // Para evitar que un drag iniciado dentro del sidebar abierto dispare un
+    // cierre inmediato, exigimos que el puntero haya salido de la franja al
+    // menos una vez tras empezar el drag. Asi: abrir = entrar a la franja
+    // tras empezar fuera (que ya pasaba); cerrar = salir de la franja y
+    // volver a entrar.
+    let hasLeftStrip = false;
     const clear = () => {
       if (timer !== null) {
         window.clearTimeout(timer);
@@ -203,16 +209,26 @@ export function App() {
       const nowInside = ev.clientX <= DRAG_EDGE_WIDTH;
       if (nowInside === inside) return;
       inside = nowInside;
-      setEdgeArmed(nowInside);
-      if (nowInside) {
-        if (navOpenRef.current) return; // already open, nothing to do
-        clear();
-        timer = window.setTimeout(() => {
-          setNavOpen(true);
-        }, DRAG_EDGE_HOVER_MS);
-      } else {
+      // El brillo visible solo cuando "armable": fuera-y-luego-dentro (open
+      // siempre) o dentro con sidebar cerrado (open trigger).
+      const armable = nowInside && (!navOpenRef.current || hasLeftStrip);
+      setEdgeArmed(armable);
+      if (!nowInside) {
+        hasLeftStrip = true;
         clear();
+        return;
       }
+      // nowInside = true
+      if (!armable) return;
+      clear();
+      const willOpen = !navOpenRef.current;
+      timer = window.setTimeout(() => {
+        setNavOpen(willOpen);
+        // Tras toggle, resetea el flag para no encadenar otra accion sin
+        // que el usuario salga + vuelva.
+        hasLeftStrip = false;
+        setEdgeArmed(false);
+      }, DRAG_EDGE_HOVER_MS);
     };
     document.addEventListener("mousemove", onMove);
     return () => {