feat(frontend): unify theming on vanilla AD-Vue light/dark/ultra-dark

The legacy panel CSS (custom.min.css ported as legacy.css) tinted every
non-primary button teal-green via .dark .ant-btn:not(.ant-btn-primary)
overrides while AD-Vue 4's darkAlgorithm kept primary buttons blue —
producing the mixed blue/green button look on dark mode. Drop legacy.css
entirely and let AD-Vue 4's algorithms own the palette.

Centralize antdThemeConfig in useTheme.js so every page resolves to the
same source of truth (light = defaultAlgorithm, dark = darkAlgorithm,
ultra-dark = darkAlgorithm + deeper colorBgBase/Layout/Container/
Elevated tokens). Each page's <a-config-provider> now imports the
shared computed instead of defining its own copy.

Drops the 67 KB legacy CSS chunk; per-page CSS bundles fall to ≤5.9 KB.

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

frontend/src/pages/settings/SettingsPage.vue

@@ -1,7 +1,7 @@
 <script setup>
-import { computed, onMounted, ref } from 'vue';
+import { onMounted, ref } from 'vue';
 import { useI18n } from 'vue-i18n';
-import { theme as antdTheme, Modal } from 'ant-design-vue';
+import { Modal } from 'ant-design-vue';
 import {
   SettingOutlined,
   SafetyOutlined,
@@ -11,7 +11,7 @@ import {
 } from '@ant-design/icons-vue';
 
 import { HttpUtil, PromiseUtil } from '@/utils';
-import { theme as themeState } from '@/composables/useTheme.js';
+import { theme as themeState, antdThemeConfig } from '@/composables/useTheme.js';
 import { useMediaQuery } from '@/composables/useMediaQuery.js';
 import AppSidebar from '@/components/AppSidebar.vue';
 import { useAllSetting } from './useAllSetting.js';
@@ -23,10 +23,6 @@ import SubscriptionFormatsTab from './SubscriptionFormatsTab.vue';
 
 const { t } = useI18n();
 
-const antdThemeConfig = computed(() => ({
-  algorithm: themeState.isDark ? antdTheme.darkAlgorithm : antdTheme.defaultAlgorithm,
-}));
-
 const { fetched, spinning, saveDisabled, allSetting, fetchAll, saveAll } = useAllSetting();
 const { isMobile } = useMediaQuery();