feat(web): management console frontend

Vue 3 + TS + Vite + Tailwind with self-built UI components. Landing,
auth, user dashboard (keys/usage/settings), and admin console
(overview/models/channels/users/usage/config). Theme system
(light/dark/system) with pre-paint boot script, theme-aware ECharts,
and a channel editor with multi-format (Anthropic/OpenAI) selection.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/components/ui/PageHeader.vue

@@ -0,0 +1,15 @@
+<script setup lang="ts">
+withDefaults(defineProps<{ title: string; desc?: string }>(), { desc: '' })
+</script>
+
+<template>
+  <div class="mb-5 flex flex-wrap items-end justify-between gap-3">
+    <div>
+      <h2 class="text-lg font-semibold text-ink">{{ title }}</h2>
+      <p v-if="desc" class="mt-1 text-sm text-ink-mute">{{ desc }}</p>
+    </div>
+    <div v-if="$slots.actions" class="flex items-center gap-2">
+      <slot name="actions" />
+    </div>
+  </div>
+</template>