feat(sidebar): pin Logout above trigger, inline 3-state theme cycle

The desktop sider stretched to match the page height, so below lg
(992px) where dashboard cards stack into one column the collapse
trigger plus Logout slid off-screen. Pin the sider with
`position: sticky; height: 100vh; align-self: flex-start` so the chrome
stays viewport-tall. Split the menu into `.sider-nav` (flex: 1,
scrollable) and `.sider-utility` so Logout sits directly above the
48px trigger reserved by padding-bottom.

Replace the `<ThemeSwitch>` a-sub-menu with a single inline icon
button next to the '3X-UI' brand (sun / moon / moon+star SVG). One
click cycles Light -> Dark -> Ultra Dark -> Light. ThemeSwitch.vue
removed since it is now inlined.

Override AD-Vue dark Menu selected + hover/active state on the
sider-nav, sider-utility, and drawer menus to use the same light-blue
tint AD-Vue's light theme uses (rgba(64,150,255,0.2) / #4096ff). The
default dark variant was too subtle against #252526, so the current
page and Logout-on-hover barely distinguished themselves.

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "3x-ui-frontend",
   "private": true,
-  "version": "0.0.1",
+  "version": "0.0.2",
   "type": "module",
   "description": "3x-ui panel frontend (Vue 3 + Ant Design Vue 4 + Vite 8).",
   "scripts": {