feat(frontend): Phase 5c-i — index.html dashboard shell

Replaces the smoke-test App.vue with a real IndexPage shell so the
/index.html route now boots the actual dashboard layout in Vue 3:

- a-config-provider drives AD-Vue 4's dark algorithm from useTheme
  (same pattern as LoginPage)
- AppSidebar (Phase 5b component) is wired in with basePath +
  requestUri props
- a-spin loading state with placeholder card while we build out the
  rest of the page
- Page palette mirrors the legacy: light #f0f2f5, dark #0a1222
  (--dark-color-background), ultra-dark #21242a

The 1,805-line legacy index.html is too big for one commit. Split
into five sub-phases on the todo list: ii) status cards + /server/status
polling, iii) xray status card, iv) logs/backup/panel-update modals,
v) custom-geo section.

frontend/src/App.vue and frontend/src/main.js (smoke-test scaffold)
are removed — both purposes now served by IndexPage and index.js.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/pages/index/IndexPage.vue

@@ -0,0 +1,95 @@
+<script setup>
+import { computed, ref } from 'vue';
+import { theme as antdTheme } from 'ant-design-vue';
+
+import { theme as themeState } from '@/composables/useTheme.js';
+import AppSidebar from '@/components/AppSidebar.vue';
+
+// Drive AD-Vue 4's built-in dark algorithm from our reactive theme.
+const antdThemeConfig = computed(() => ({
+  algorithm: themeState.isDark ? antdTheme.darkAlgorithm : antdTheme.defaultAlgorithm,
+}));
+
+// Phase 5c-i ships the page shell only — sidebar, layout, theme.
+// Real content (CPU/mem/swap/disk cards, Xray status card, panel
+// update modal, logs, custom-geo section) follows in 5c-ii through
+// 5c-iv. Loading state is currently a placeholder true so the shell
+// renders; it will be wired to the real /server/status fetch later.
+const fetched = ref(true);
+
+// In production the Go panel injects basePath + requestUri into the
+// served HTML; during `npm run dev` we infer them from window.location.
+const basePath = window.__X_UI_BASE_PATH__ || '';
+const requestUri = window.location.pathname;
+</script>
+
+<template>
+  <a-config-provider :theme="antdThemeConfig">
+    <a-layout class="index-page" :class="{ 'is-dark': themeState.isDark, 'is-ultra': themeState.isUltra }">
+      <AppSidebar :base-path="basePath" :request-uri="requestUri" />
+
+      <a-layout class="content-shell">
+        <a-layout-content class="content-area">
+          <a-spin :spinning="!fetched" :delay="200" size="large">
+            <div v-if="!fetched" class="loading-spacer" />
+
+            <div v-else class="page-body">
+              <a-card hoverable>
+                <a-space direction="vertical" :size="12" style="width: 100%">
+                  <h2 style="margin: 0">Dashboard (vue3-migration shell)</h2>
+                  <p style="margin: 0; opacity: 0.7">
+                    Phase 5c-i: layout, sidebar, and theme switching wired up.
+                    Status cards, xray controls, and custom-geo arrive in
+                    follow-up commits.
+                  </p>
+                </a-space>
+              </a-card>
+            </div>
+          </a-spin>
+        </a-layout-content>
+      </a-layout>
+    </a-layout>
+  </a-config-provider>
+</template>
+
+<style scoped>
+.index-page {
+  /* Same legacy palette source as the login page. */
+  --bg-page: #f0f2f5;
+  --bg-card: #ffffff;
+
+  min-height: 100vh;
+  background: var(--bg-page);
+}
+
+.index-page.is-dark {
+  --bg-page: #0a1222;  /* legacy --dark-color-background */
+  --bg-card: #151f31;  /* legacy --dark-color-surface-100 */
+}
+
+.index-page.is-dark.is-ultra {
+  --bg-page: #21242a;  /* legacy ultra --dark-color-background */
+  --bg-card: #0c0e12;  /* legacy ultra surface-100 */
+}
+
+.index-page :deep(.ant-layout),
+.index-page :deep(.ant-layout-content) {
+  background: transparent;
+}
+
+.content-shell {
+  background: transparent;
+}
+
+.content-area {
+  padding: 24px;
+}
+
+.loading-spacer {
+  min-height: calc(100vh - 120px);
+}
+
+.page-body :deep(.ant-card) {
+  background: var(--bg-card);
+}
+</style>