feat(export): add .toml source download and shared slugify helper

src/composables/useSourceExport.ts

@@ -0,0 +1,17 @@
+import { slugify } from './usePngExport'
+
+/** Slugified, stable download name for a plan's TOML source. */
+export function sourceFilename(title: string): string {
+  return `macroplan-${slugify(title) || 'plan'}.toml`
+}
+
+/** Download a plan's TOML source as a .toml file (client-side, no backend). */
+export function downloadSource(source: string, filename: string): void {
+  const blob = new Blob([source], { type: 'text/plain;charset=utf-8' })
+  const url = URL.createObjectURL(blob)
+  const a = document.createElement('a')
+  a.href = url
+  a.download = filename
+  a.click()
+  URL.revokeObjectURL(url)
+}