Fix login: parse API error body into Error.message

api.post/get/put/delete now throw Error with .message from response
body (detail/message field), not raw Response object. Fixes login
modal showing no feedback on auth failure.

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

frontend/src/composables/useApi.js

@@ -24,7 +24,16 @@ async function request(path, opts = {}) {
 
 async function requestJSON(path, opts = {}) {
   const res = await request(path, opts)
-  if (!res.ok) throw res
+  if (!res.ok) {
+    let msg = `${res.status}`
+    try {
+      const body = await res.json()
+      msg = body.detail || body.message || msg
+    } catch {}
+    const err = new Error(msg)
+    err.status = res.status
+    throw err
+  }
   return res.json()
 }