Resume pending action after login (favorite, diary, QR upload)

Add pendingAction callback to UI store. When user clicks favorite,
save-to-diary, or upload QR while not logged in, the action is stored
and automatically executed after successful login/register instead of
reloading the page.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/RecipeDetailOverlay.vue

@@ -490,7 +490,7 @@ const showBrandHint = computed(() =>
 
 function goUploadQr() {
   if (!authStore.isLoggedIn) {
-    ui.openLogin()
+    ui.openLogin(() => goUploadQr())
     return
   }
   if (recipe.value._id) {
@@ -606,7 +606,7 @@ function getCardRecipeName() {
 // ---- Favorite ----
 async function handleToggleFavorite() {
   if (!authStore.isLoggedIn) {
-    ui.openLogin()
+    ui.openLogin(() => handleToggleFavorite())
     return
   }
   if (!recipe.value._id) {
@@ -625,7 +625,7 @@ async function handleToggleFavorite() {
 // ---- Save to diary ----
 async function saveToDiary() {
   if (!authStore.isLoggedIn) {
-    ui.openLogin()
+    ui.openLogin(() => saveToDiary())
     return
   }
   const name = prompt('保存为我的配方，名称：', recipe.value.name)