feat(03-05): 3-step presigned upload + quota state in auth store + progress UI

- api/client.js: extend request() to attach .status and .payload on 413 structured errors; remove legacy uploadDocument multipart function
- stores/auth.js: add quota ref({used_bytes:0, limit_bytes:0}) and fetchQuota() action (silent catch); expose in store return
- stores/documents.js: replace single upload() with uploadToMinIO XHR helper + 3-step async action (getUploadUrl→XHR PUT→confirmUpload); track uploadProgress map keyed by filename+timestamp (T-03-25); call fetchQuota after upload success and document delete
- components/upload/UploadProgress.vue: add aria progressbar per row, percentage label, quota rejection error block (role=alert, red-50/red-200) from item.quotaError; use plain anchor for Manage storage link

frontend/src/stores/auth.js

@@ -22,6 +22,7 @@ export const useAuthStore = defineStore('auth', () => {
   const user = ref(null)
   const loading = ref(false)
   const error = ref(null)
+  const quota = ref({ used_bytes: 0, limit_bytes: 0 })
 
   /**
    * Register a new account.
@@ -122,15 +123,30 @@ export const useAuthStore = defineStore('auth', () => {
     }
   }
 
+  /**
+   * Fetch current user quota from the server and update quota ref.
+   * Silently ignores errors — QuotaBar handles error display by hiding itself (UI-SPEC).
+   */
+  async function fetchQuota() {
+    try {
+      const data = await api.getMyQuota()
+      quota.value = { used_bytes: data.used_bytes, limit_bytes: data.limit_bytes }
+    } catch {
+      // Silently ignore — QuotaBar hides itself on fetch error (UI-SPEC)
+    }
+  }
+
   return {
     accessToken,
     user,
     loading,
     error,
+    quota,
     register,
     login,
     logout,
     logoutAll,
     refresh,
+    fetchQuota,
   }
 })