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

src/app/components/ResizeHandle.module.css

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+/*
+ * 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);
+}

src/app/components/ResizeHandle.tsx

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+/**
+ * Vertical drag handle between two panes (spec §01A).
+ *
+ * Sits between a side pane and the editor; dragging resizes the side pane while
+ * the editor absorbs the change, so the opposite side pane is unaffected. Width
+ * is clamped (pure `clampSideWidth`) so neither the dragged pane nor the editor
+ * falls below its minimum. Keyboard accessible: focus and use ←/→ to nudge.
+ *
+ * The handle reads the panes-row width from its own parent at interaction time,
+ * so it needs no layout props — it just controls the `side` it is told to.
+ */
+
+import { useRef } from 'react';
+import { clampSideWidth, usePanesStore, type PaneSide } from '../stores/PanesStore';
+import styles from './ResizeHandle.module.css';
+
+/** Keyboard nudge step (px) per arrow press. */
+const KEY_STEP = 16;
+
+interface ResizeHandleProps {
+  /** Which side pane this handle resizes. */
+  side: PaneSide;
+  /** Accessible label, e.g. "Resize snippet library". */
+  label: string;
+}
+
+export function ResizeHandle({ side, label }: ResizeHandleProps) {
+  const ref = useRef<HTMLDivElement>(null);
+
+  /** Full panes-row width — the handle's parent (`.panes`). */
+  const containerWidth = (): number => ref.current?.parentElement?.clientWidth ?? 0;
+
+  /** Apply a desired width for this side, clamped against the current layout. */
+  const applyWidth = (desired: number) => {
+    const { libraryWidth, previewWidth, setWidth } = usePanesStore.getState();
+    const other = side === 'library' ? previewWidth : libraryWidth;
+    setWidth(side, clampSideWidth(side, desired, containerWidth(), other));
+  };
+
+  const onPointerDown = (e: React.PointerEvent<HTMLDivElement>) => {
+    if (e.button !== 0) return; // primary button only
+    e.preventDefault();
+    const startX = e.clientX;
+    const startWidth =
+      side === 'library'
+        ? usePanesStore.getState().libraryWidth
+        : usePanesStore.getState().previewWidth;
+
+    const onMove = (ev: PointerEvent) => {
+      const delta = ev.clientX - startX;
+      // The left handle grows its pane as it moves right; the right handle (left
+      // of the preview) shrinks the preview as it moves right.
+      const desired = side === 'library' ? startWidth + delta : startWidth - delta;
+      applyWidth(desired);
+    };
+    const onUp = () => {
+      window.removeEventListener('pointermove', onMove);
+      window.removeEventListener('pointerup', onUp);
+      document.body.style.cursor = '';
+      document.body.style.userSelect = '';
+    };
+
+    window.addEventListener('pointermove', onMove);
+    window.addEventListener('pointerup', onUp);
+    // While dragging, force the resize cursor and suppress text selection.
+    document.body.style.cursor = 'col-resize';
+    document.body.style.userSelect = 'none';
+  };
+
+  const onKeyDown = (e: React.KeyboardEvent<HTMLDivElement>) => {
+    if (e.key !== 'ArrowLeft' && e.key !== 'ArrowRight') return;
+    e.preventDefault();
+    const dir = e.key === 'ArrowRight' ? 1 : -1;
+    const current =
+      side === 'library'
+        ? usePanesStore.getState().libraryWidth
+        : usePanesStore.getState().previewWidth;
+    const delta = side === 'library' ? dir * KEY_STEP : -dir * KEY_STEP;
+    applyWidth(current + delta);
+  };
+
+  return (
+    // TODO: a focusable window-splitter should also expose aria-valuenow/min/max
+    // (the side pane's current/min/max width) so assistive tech can announce the
+    // size as it changes. Wire it when the pane toggle strip lands in M6.
+    <div
+      ref={ref}
+      className={styles.handle}
+      role="separator"
+      aria-orientation="vertical"
+      aria-label={label}
+      tabIndex={0}
+      onPointerDown={onPointerDown}
+      onKeyDown={onKeyDown}
+    >
+      <span className={styles.grip} aria-hidden="true" />
+    </div>
+  );
+}