Add resizable panes with drag handles, min widths, and persistence

src/app/components/ResizeHandle.module.css

@@ -0,0 +1,35 @@
+/*
+ * 6px-wide hit target with a thin visible grip. Width is part of the layout
+ * budget (HANDLES_TOTAL in PanesStore); keep them in sync if this changes.
+ */
+.handle {
+  flex: 0 0 6px;
+  position: relative;
+  cursor: col-resize;
+  background: var(--border);
+  touch-action: none; /* let pointer drags own the gesture, not scroll */
+  transition: background var(--dur-fast) var(--ease);
+}
+
+.handle:hover,
+.handle:focus-visible {
+  background: var(--accent);
+  outline: none;
+}
+
+/* A short centered grip line, so the handle reads as a draggable divider. */
+.grip {
+  position: absolute;
+  top: 50%;
+  left: 50%;
+  width: 2px;
+  height: 24px;
+  transform: translate(-50%, -50%);
+  background: var(--border-strong);
+  border-radius: var(--radius);
+}
+
+.handle:hover .grip,
+.handle:focus-visible .grip {
+  background: var(--accent-contrast);
+}