feat(ui): draggable Splitter component for resizable panes

WAI-ARIA "window splitter" pattern (role=separator, keyboard-operable) --
reusable divider used by the resizable/hideable pane layout.

src/lib/components/Splitter.svelte

@@ -0,0 +1,105 @@
+<script lang="ts">
+  // Draggable divider between two panes (FR-UX-1). Reports a delta in
+  // pixels via onResize as the pointer moves; the parent owns the actual
+  // size state and clamping. onResizeEnd fires once per drag (and per arrow
+  // keypress) so the parent can persist without writing on every pixel.
+  let {
+    orientation = "vertical",
+    onResize,
+    onResizeEnd,
+    label,
+  }: {
+    orientation?: "vertical" | "horizontal";
+    onResize: (deltaPx: number) => void;
+    onResizeEnd?: () => void;
+    label: string;
+  } = $props();
+
+  let dragging = $state(false);
+  let lastPos = 0;
+
+  function posOf(e: PointerEvent): number {
+    return orientation === "vertical" ? e.clientX : e.clientY;
+  }
+
+  function onPointerDown(e: PointerEvent) {
+    dragging = true;
+    lastPos = posOf(e);
+    (e.currentTarget as HTMLElement).setPointerCapture(e.pointerId);
+  }
+  function onPointerMove(e: PointerEvent) {
+    if (!dragging) return;
+    const pos = posOf(e);
+    onResize(pos - lastPos);
+    lastPos = pos;
+  }
+  function onPointerUp(e: PointerEvent) {
+    if (!dragging) return;
+    dragging = false;
+    (e.currentTarget as HTMLElement).releasePointerCapture(e.pointerId);
+    onResizeEnd?.();
+  }
+  function onKeydown(e: KeyboardEvent) {
+    const step = e.shiftKey ? 40 : 12;
+    const negKey = orientation === "vertical" ? "ArrowLeft" : "ArrowUp";
+    const posKey = orientation === "vertical" ? "ArrowRight" : "ArrowDown";
+    if (e.key === negKey) onResize(-step);
+    else if (e.key === posKey) onResize(step);
+    else return;
+    e.preventDefault();
+    onResizeEnd?.();
+  }
+</script>
+
+<!-- WAI-ARIA "window splitter" pattern: a focusable, keyboard-operable
+     role="separator" is the correct/standard shape for a resize handle —
+     the a11y linter's generic "non-interactive element" rule doesn't know
+     about this pattern specifically. -->
+<!-- svelte-ignore a11y_no_noninteractive_tabindex -->
+<!-- svelte-ignore a11y_no_noninteractive_element_interactions -->
+<div
+  class="splitter {orientation}"
+  class:dragging
+  role="separator"
+  aria-orientation={orientation}
+  aria-label={label}
+  tabindex="0"
+  onpointerdown={onPointerDown}
+  onpointermove={onPointerMove}
+  onpointerup={onPointerUp}
+  onkeydown={onKeydown}
+></div>
+
+<style>
+  .splitter {
+    flex: none;
+    background: transparent;
+    position: relative;
+  }
+  .splitter.vertical {
+    width: 5px;
+    cursor: col-resize;
+  }
+  .splitter.horizontal {
+    height: 5px;
+    cursor: row-resize;
+  }
+  /* Wider invisible hit-area than the visible line — a 5px target is too
+     thin to reliably grab (touch-target-size / no-precision-required). */
+  .splitter::after {
+    content: "";
+    position: absolute;
+  }
+  .splitter.vertical::after {
+    inset: 0 -4px;
+  }
+  .splitter.horizontal::after {
+    inset: -4px 0;
+  }
+  .splitter:hover,
+  .splitter:focus-visible,
+  .splitter.dragging {
+    background: var(--accent-soft);
+    outline: none;
+  }
+</style>