feat(08-03): create useToastStore Pinia stub with locked show() contract

- Defines show(message, type='success', duration=4000) as silent no-op
- Positional parameters only per UI-SPEC.md contract
- Phase 10 (UX-10) must implement rendering without modifying call sites

frontend/src/stores/toast.js

@@ -0,0 +1,20 @@
+/**
+ * Phase 7.1 STUB — Phase 10 (UX-10) fills in the full implementation.
+ *
+ * The signature `show(message, type, duration)` is locked and Phase 10 must
+ * honor it without modifying Phase 8 call sites.
+ *
+ * Default values match UI-SPEC.md:
+ *   type     = 'success'  (NOT 'info')
+ *   duration = 4000
+ */
+import { defineStore } from 'pinia'
+
+export const useToastStore = defineStore('toast', () => {
+  // eslint-disable-next-line no-unused-vars
+  function show(message, type = 'success', duration = 4000) {
+    // No-op stub — Phase 10 implements rendering.
+  }
+
+  return { show }
+})