feat(03-04): replace settings UI with admin-managed placeholder; update API client

- views/SettingsView.vue: Replace full form with static placeholder card. No store
  imports, no API calls. Shows "AI configuration is managed by your administrator."
  (D-12, T-03-21)
- stores/settings.js: Deleted — only consumed by SettingsView; no other imports
- api/client.js: Remove getSettings, patchSettings, testProvider, getDefaultPrompt
  (// Settings section deleted). Add getMyQuota() for quota bar (Plan 03-05).
  Add getUploadUrl() and confirmUpload() for presigned upload flow (Plan 03-05).

frontend/src/stores/settings.js

@@ -1,38 +0,0 @@
-import { defineStore } from 'pinia'
-import { ref } from 'vue'
-import * as api from '../api/client.js'
-
-export const useSettingsStore = defineStore('settings', () => {
-  const settings = ref(null)
-  const loading = ref(false)
-  const error = ref(null)
-
-  async function fetchSettings() {
-    loading.value = true
-    error.value = null
-    try {
-      settings.value = await api.getSettings()
-    } catch (e) {
-      error.value = e.message
-    } finally {
-      loading.value = false
-    }
-  }
-
-  async function save(patch) {
-    const updated = await api.patchSettings(patch)
-    settings.value = updated
-    return updated
-  }
-
-  async function testConnection(provider) {
-    return api.testProvider(provider)
-  }
-
-  async function resetPrompt() {
-    const data = await api.getDefaultPrompt()
-    return data.system_prompt
-  }
-
-  return { settings, loading, error, fetchSettings, save, testConnection, resetPrompt }
-})