Add M3: APPDATA persistence + presets + per-pane distro/label

Backend:
- save_workspace / load_workspace Tauri commands writing to
  %APPDATA%\com.megaproxy.tiletopia\workspace.json with atomic
  tmp+rename. Path from app.path().app_config_dir() (no dirs crate).

Layout helpers:
- tree.ts: changeDistro (with id swap to force XtermPane remount via
  {#key}), changeLabel, presetSingle / TwoColumns / ThreeColumns /
  TwoRows / TwoByTwo.
- New ops.ts with PaneOps interface bundling split / close /
  setDistro / setLabel / distros, drilled through Pane chain
  instead of individual callbacks.

UI:
- LeafPane: in-toolbar editable label (click to rename, Enter
  saves, Esc cancels) and distro chip popover. Picking a different
  distro respawns the pane.
- App.svelte: migrated from localStorage to APPDATA via the new
  Tauri commands, debounced 500ms. One-time localStorage migration
  on boot. Split inherits parent's distro+cwd. Titlebar preset
  buttons with confirm when replacing >1 pane.

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

src/lib/layout/tree.ts

@@ -108,6 +108,66 @@ export function findLeaf(root: TreeNode, leafId: NodeId): LeafNode | null {
   return findLeaf(root.a, leafId) ?? findLeaf(root.b, leafId);
 }
 
+/**
+ * Swap the distro on a leaf. The leaf gets a **new id** so the rendering
+ * layer's `{#key node.id}` block remounts XtermPane — the old PTY is killed
+ * and a fresh one spawns with the new distro.
+ */
+export function changeDistro(
+  root: TreeNode,
+  leafId: NodeId,
+  distro: string,
+): TreeNode {
+  return replaceById(root, leafId, (node) => {
+    if (node.kind !== "leaf") return node;
+    return { ...node, id: newId(), distro };
+  });
+}
+
+/** Set or clear a leaf's label. Does NOT remount (label is metadata only). */
+export function changeLabel(
+  root: TreeNode,
+  leafId: NodeId,
+  label: string | undefined,
+): TreeNode {
+  return replaceById(root, leafId, (node) => {
+    if (node.kind !== "leaf") return node;
+    const trimmed = label?.trim();
+    return { ...node, label: trimmed ? trimmed : undefined };
+  });
+}
+
+// ---- preset layouts --------------------------------------------------------
+
+type LeafDefaults = Partial<Omit<LeafNode, "kind" | "id">>;
+
+export function presetSingle(d: LeafDefaults = {}): TreeNode {
+  return newLeaf(d);
+}
+
+export function presetTwoColumns(d: LeafDefaults = {}): TreeNode {
+  return newSplit("h", newLeaf(d), newLeaf(d));
+}
+
+export function presetThreeColumns(d: LeafDefaults = {}): TreeNode {
+  // Even thirds: outer split at 1/3, inner split at 1/2.
+  return newSplit(
+    "h",
+    newLeaf(d),
+    newSplit("h", newLeaf(d), newLeaf(d), 0.5),
+    1 / 3,
+  );
+}
+
+export function presetTwoRows(d: LeafDefaults = {}): TreeNode {
+  return newSplit("v", newLeaf(d), newLeaf(d));
+}
+
+export function presetTwoByTwo(d: LeafDefaults = {}): TreeNode {
+  const row = () => newSplit("h", newLeaf(d), newLeaf(d));
+  return newSplit("v", row(), row());
+}
+
 /** Number of leaves in the tree. */
 export function leafCount(root: TreeNode): number {
   if (root.kind === "leaf") return 1;