fix(v2): rename rune stores to .svelte.ts to fix rune_outside_svelte error

Svelte 5 $state/$derived runes only work in .svelte and .svelte.ts
files. The stores had plain .ts extensions, causing a blank screen with
"rune_outside_svelte" runtime error. Renamed all three store files and
updated import paths across 5 consuming files.

v2/src/lib/stores/layout.svelte.ts

@@ -0,0 +1,90 @@
+export type LayoutPreset = '1-col' | '2-col' | '3-col' | '2x2' | 'master-stack';
+
+export type PaneType = 'terminal' | 'agent' | 'markdown' | 'empty';
+
+export interface Pane {
+  id: string;
+  type: PaneType;
+  title: string;
+  shell?: string;
+  cwd?: string;
+  args?: string[];
+  focused: boolean;
+}
+
+let panes = $state<Pane[]>([]);
+let activePreset = $state<LayoutPreset>('1-col');
+let focusedPaneId = $state<string | null>(null);
+
+export function getPanes(): Pane[] {
+  return panes;
+}
+
+export function getActivePreset(): LayoutPreset {
+  return activePreset;
+}
+
+export function getFocusedPaneId(): string | null {
+  return focusedPaneId;
+}
+
+export function addPane(pane: Omit<Pane, 'focused'>): void {
+  panes.push({ ...pane, focused: false });
+  focusPane(pane.id);
+  autoPreset();
+}
+
+export function removePane(id: string): void {
+  panes = panes.filter(p => p.id !== id);
+  if (focusedPaneId === id) {
+    focusedPaneId = panes.length > 0 ? panes[0].id : null;
+  }
+  autoPreset();
+}
+
+export function focusPane(id: string): void {
+  focusedPaneId = id;
+  panes = panes.map(p => ({ ...p, focused: p.id === id }));
+}
+
+export function focusPaneByIndex(index: number): void {
+  if (index >= 0 && index < panes.length) {
+    focusPane(panes[index].id);
+  }
+}
+
+export function setPreset(preset: LayoutPreset): void {
+  activePreset = preset;
+}
+
+function autoPreset(): void {
+  const count = panes.length;
+  if (count <= 1) activePreset = '1-col';
+  else if (count === 2) activePreset = '2-col';
+  else if (count === 3) activePreset = 'master-stack';
+  else activePreset = '2x2';
+}
+
+/** CSS grid-template for current preset */
+export function getGridTemplate(): { columns: string; rows: string } {
+  switch (activePreset) {
+    case '1-col':
+      return { columns: '1fr', rows: '1fr' };
+    case '2-col':
+      return { columns: '1fr 1fr', rows: '1fr' };
+    case '3-col':
+      return { columns: '1fr 1fr 1fr', rows: '1fr' };
+    case '2x2':
+      return { columns: '1fr 1fr', rows: '1fr 1fr' };
+    case 'master-stack':
+      return { columns: '2fr 1fr', rows: '1fr 1fr' };
+  }
+}
+
+/** For master-stack: first pane spans full height */
+export function getPaneGridArea(index: number): string | undefined {
+  if (activePreset === 'master-stack' && index === 0) {
+    return '1 / 1 / 3 / 2';
+  }
+  return undefined;
+}