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/stores/ui.js

@@ -5,6 +5,7 @@ export const useUiStore = defineStore('ui', () => {
   const currentSection = ref('search')
   const showLoginModal = ref(false)
   const toasts = ref([])
+  const pendingAction = ref(null)
 
   let toastId = 0
 
@@ -20,7 +21,10 @@ export const useUiStore = defineStore('ui', () => {
     }, duration)
   }
 
-  function openLogin() {
+  function openLogin(afterLogin) {
+    if (afterLogin) {
+      pendingAction.value = afterLogin
+    }
     showLoginModal.value = true
   }
 
@@ -28,13 +32,23 @@ export const useUiStore = defineStore('ui', () => {
     showLoginModal.value = false
   }
 
+  function runPendingAction() {
+    if (pendingAction.value) {
+      const action = pendingAction.value
+      pendingAction.value = null
+      action()
+    }
+  }
+
   return {
     currentSection,
     showLoginModal,
     toasts,
+    pendingAction,
     showSection,
     showToast,
     openLogin,
     closeLogin,
+    runPendingAction,
   }
 })