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/api/client.js

@@ -34,21 +34,26 @@ async function request(path, options = {}) {
 
   if (!res.ok) {
     let msg = `HTTP ${res.status}`
-    try { msg = (await res.json()).detail || msg } catch {}
-    throw new Error(msg)
+    let payload = null
+    try {
+      const body = await res.json()
+      if (typeof body.detail === 'object' && body.detail !== null) {
+        payload = body.detail
+        msg = body.detail.message || `HTTP ${res.status}`
+      } else {
+        msg = body.detail || msg
+      }
+    } catch {}
+    const err = new Error(msg)
+    err.status = res.status
+    if (payload) err.payload = payload
+    throw err
   }
   return res.json()
 }
 
 // ── Documents ────────────────────────────────────────────────────────────────
 
-export function uploadDocument(file, autoClassify = true) {
-  const form = new FormData()
-  form.append('file', file)
-  form.append('auto_classify', autoClassify ? 'true' : 'false')
-  return request('/api/documents/upload', { method: 'POST', body: form })
-}
-
 export function listDocuments({ topic, page = 1, perPage = 20 } = {}) {
   const params = new URLSearchParams({ page, per_page: perPage })
   if (topic) params.set('topic', topic)