feat(layout): toggle between source, macroplan, and split panes

The chosen layout is kept in the URL (?view=) so it's shareable and
bookmarkable rather than persisted. PNG export is disabled in
source-only mode since the rendered node it captures isn't mounted then.

src/App.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { ref, computed } from "vue"
+import { ref, computed, watch } from "vue"
 import { useMacroplan } from "./composables/useMacroplan"
 import { usePngExport, exportFilename } from "./composables/usePngExport"
 import { sourceFilename, downloadSource } from "./composables/useSourceExport"
@@ -13,6 +13,28 @@ const { busy, toast, copyPng, downloadPng } = usePngExport()
 const exportRoot = ref<HTMLElement>()
 const confirmingDelete = ref(false)
 
+// Which panes are visible: the Source editor, the rendered Macroplan, or both.
+// Kept in the URL (?view=…) so a layout is shareable/bookmarkable, not persisted.
+type ViewMode = "source" | "split" | "view"
+
+function readViewMode(): ViewMode {
+  const v = new URLSearchParams(location.search).get("view")
+  return v === "source" || v === "view" ? v : "split"
+}
+
+const mode = ref<ViewMode>(readViewMode())
+const showSource = computed(() => mode.value !== "view")
+const showView = computed(() => mode.value !== "source")
+
+// Reflect the choice in the URL without stacking a history entry per toggle.
+// "split" is the default, so drop the param to keep the URL clean.
+watch(mode, (m) => {
+  const url = new URL(location.href)
+  if (m === "split") url.searchParams.delete("view")
+  else url.searchParams.set("view", m)
+  history.replaceState(history.state, "", url)
+})
+
 const activeName = computed(
   () => plans.value.find((p) => p.id === activeId.value)?.name ?? "Untitled",
 )
@@ -40,9 +62,38 @@ function confirmDelete() {
           @download="downloadToml"
         />
       </div>
+      <div class="join" role="group" aria-label="Choose which panes to show">
+        <button
+          class="btn btn-sm join-item"
+          :class="{ 'btn-active': mode === 'source' }"
+          :aria-pressed="mode === 'source'"
+          title="Show only the TOML source"
+          @click="mode = 'source'"
+        >
+          Source
+        </button>
+        <button
+          class="btn btn-sm join-item"
+          :class="{ 'btn-active': mode === 'split' }"
+          :aria-pressed="mode === 'split'"
+          title="Show the source and the Macroplan side by side"
+          @click="mode = 'split'"
+        >
+          Split
+        </button>
+        <button
+          class="btn btn-sm join-item"
+          :class="{ 'btn-active': mode === 'view' }"
+          :aria-pressed="mode === 'view'"
+          title="Show only the rendered Macroplan"
+          @click="mode = 'view'"
+        >
+          Macroplan
+        </button>
+      </div>
       <button
         class="btn btn-ghost btn-sm"
-        :disabled="!plan || busy"
+        :disabled="!plan || busy || !showView"
         title="Copy the rendered plan to the clipboard as a PNG"
         @click="copyPng(exportRoot)"
       >
@@ -51,7 +102,7 @@ function confirmDelete() {
       </button>
       <button
         class="btn btn-ghost btn-sm"
-        :disabled="!plan || busy"
+        :disabled="!plan || busy || !showView"
         title="Download the rendered plan as a PNG"
         @click="downloadPng(exportRoot, exportFilename(plan?.title ?? ''))"
       >
@@ -69,7 +120,11 @@ function confirmDelete() {
 
     <main class="flex min-h-0 flex-1 flex-col md:flex-row">
       <section
-        class="flex min-h-0 flex-col border-base-300 max-md:h-2/5 max-md:border-b md:w-1/3 md:max-w-md md:border-r"
+        v-if="showSource"
+        class="flex min-h-0 flex-col border-base-300"
+        :class="
+          showView ? 'max-md:h-2/5 max-md:border-b md:w-1/3 md:max-w-md md:border-r' : 'flex-1'
+        "
       >
         <div class="px-4 py-2 text-xs font-semibold uppercase tracking-wide text-base-content/50">
           Source (TOML)
@@ -77,7 +132,7 @@ function confirmDelete() {
         <PlanEditor v-model="source" :error="error" class="min-h-0 flex-1" />
       </section>
 
-      <section class="min-h-0 flex-1 overflow-auto p-4">
+      <section v-if="showView" class="min-h-0 flex-1 overflow-auto p-4">
         <div v-if="plan" ref="exportRoot" class="export-root">
           <h2 class="mb-3 text-sm font-semibold text-base-content/70">{{ plan.title }}</h2>
           <MacroplanGrid :plan="plan" />