refactor: M5 useQuotaNotifyState composable + H14 Vue file splits

M5: New composable frontend/src/composables/useQuotaNotifyState.ts
  - Replaces 9 individual refs in both Create/Edit modals with reactive state
  - Provides loadFromExtra/writeToExtra/reset helpers
  - Eliminates ~120 lines of duplicated code across the two modals

H14: Vue file length violations fixed
  - AdminPaymentPlansView.vue: 325 → 183 lines (extracted PlanEditDialog.vue)
  - QuotaLimitCard.vue: 327 → 268 lines (extracted QuotaDimensionRow.vue)
  - PlanEditDialog.vue: 181 lines (new, plan create/edit form)
  - QuotaDimensionRow.vue: 108 lines (new, single quota dimension row)

frontend/src/composables/useQuotaNotifyState.ts

@@ -0,0 +1,69 @@
+import { reactive, ref } from 'vue'
+import { adminAPI } from '@/api/admin'
+import { QUOTA_THRESHOLD_TYPE_FIXED } from '@/constants/account'
+
+export const QUOTA_NOTIFY_DIMS = ['daily', 'weekly', 'total'] as const
+export type QuotaNotifyDim = (typeof QUOTA_NOTIFY_DIMS)[number]
+
+interface DimState {
+  enabled: boolean | null
+  threshold: number | null
+  thresholdType: string | null
+}
+
+export function useQuotaNotifyState() {
+  const globalEnabled = ref(false)
+  const state = reactive<Record<QuotaNotifyDim, DimState>>({
+    daily: { enabled: null, threshold: null, thresholdType: null },
+    weekly: { enabled: null, threshold: null, thresholdType: null },
+    total: { enabled: null, threshold: null, thresholdType: null },
+  })
+
+  function loadGlobalState() {
+    adminAPI.settings
+      .getSettings()
+      .then((settings) => {
+        globalEnabled.value = settings.account_quota_notify_enabled === true
+      })
+      .catch(() => {
+        globalEnabled.value = false
+      })
+  }
+
+  function loadFromExtra(extra: Record<string, unknown> | null | undefined) {
+    for (const d of QUOTA_NOTIFY_DIMS) {
+      state[d].enabled = (extra?.[`quota_notify_${d}_enabled`] as boolean) ?? null
+      state[d].threshold = (extra?.[`quota_notify_${d}_threshold`] as number) ?? null
+      state[d].thresholdType = (extra?.[`quota_notify_${d}_threshold_type`] as string) ?? null
+    }
+  }
+
+  function writeToExtra(extra: Record<string, unknown>, mode: 'create' | 'update') {
+    for (const d of QUOTA_NOTIFY_DIMS) {
+      const s = state[d]
+      if (s.enabled) {
+        extra[`quota_notify_${d}_enabled`] = true
+        if (s.threshold != null) {
+          extra[`quota_notify_${d}_threshold`] = s.threshold
+        } else if (mode === 'update') {
+          delete extra[`quota_notify_${d}_threshold`]
+        }
+        extra[`quota_notify_${d}_threshold_type`] = s.thresholdType || QUOTA_THRESHOLD_TYPE_FIXED
+      } else if (mode === 'update') {
+        delete extra[`quota_notify_${d}_enabled`]
+        delete extra[`quota_notify_${d}_threshold`]
+        delete extra[`quota_notify_${d}_threshold_type`]
+      }
+    }
+  }
+
+  function reset() {
+    for (const d of QUOTA_NOTIFY_DIMS) {
+      state[d].enabled = null
+      state[d].threshold = null
+      state[d].thresholdType = null
+    }
+  }
+
+  return { globalEnabled, state, loadGlobalState, loadFromExtra, writeToExtra, reset }
+}