refactor(frontend): comprehensive split of large view files into modular components

- Split UsersView.vue into UserCreateModal, UserEditModal, UserApiKeysModal, etc.
- Split UsageView.vue into UsageStatsCards, UsageFilters, UsageTable, etc.
- Split DashboardView.vue into UserDashboardStats, UserDashboardCharts, etc.
- Split AccountsView.vue into AccountTableActions, AccountTableFilters, etc.
- Standardized TypeScript types across new components to resolve implicit 'any' and 'never[]' errors.
- Improved overall frontend maintainability and code clarity.

frontend/src/components/common/Skeleton.vue

@@ -0,0 +1,46 @@
+<template>
+  <div
+    :class="[
+      'animate-pulse bg-gray-200 dark:bg-dark-700',
+      variant === 'circle' ? 'rounded-full' : 'rounded-lg',
+      customClass
+    ]"
+    :style="style"
+  ></div>
+</template>
+
+<script setup lang="ts">
+import { computed } from 'vue'
+
+interface Props {
+  variant?: 'rect' | 'circle' | 'text'
+  width?: string | number
+  height?: string | number
+  class?: string
+}
+
+const props = withDefaults(defineProps<Props>(), {
+  variant: 'rect',
+  width: '100%'
+})
+
+const customClass = computed(() => props.class || '')
+
+const style = computed(() => {
+  const s: Record<string, string> = {}
+  
+  if (props.width) {
+    s.width = typeof props.width === 'number' ? `${props.width}px` : props.width
+  }
+  
+  if (props.height) {
+    s.height = typeof props.height === 'number' ? `${props.height}px` : props.height
+  } else if (props.variant === 'text') {
+    s.height = '1em'
+    s.marginTop = '0.25em'
+    s.marginBottom = '0.25em'
+  }
+  
+  return s
+})
+</script>