refactor: remove Tauri auto-updater, replace with simple GitHub-check modal

Remove tauri-plugin-updater, relaunch_after_update command, generate-update-
manifest.js, the generate-manifest CI job, and all signing/upload steps for
.sig files and latest.json. The updater plugin, its pubkey and endpoint config
in tauri.conf.json, and the updater:default capability are all gone.

Replace AppUpdater.tsx with a lightweight component that:
- Fetches https://api.github.com/repos/Psychotoxical/psysonic/releases/latest
  after a 4 s delay (no install, no download, no progress bar)
- Shows a dismissible toast with two buttons:
  GitHub → releases/latest
  Website → https://psysonic.psychotoxic.eu/#downloads

i18n: remove updaterInstall/Downloading/Installing/Download/ExperimentalHint
keys from all 7 locales; add updaterWebsite; update updaterVersion wording.

CI: remove generate-manifest job and all .sig signing/upload steps from
build-macos-windows. Also remove TAURI_SIGNING_PRIVATE_KEY env vars (no
longer needed). Release now just builds and uploads the installable binaries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/zh.ts

@@ -15,9 +15,6 @@ export const zhTranslation = {
     help: '帮助',
     expand: '展开侧边栏',
     collapse: '收起侧边栏',
-    updateAvailable: '有可用更新',
-    updateReady: '{{version}} 已就绪',
-    updateLink: '前往发布页面 →',
     downloadingTracks: '正在缓存 {{n}} 首歌曲…',
     offlineLibrary: '离线音乐库',
     genres: '流派',
@@ -324,12 +321,8 @@ export const zhTranslation = {
     bulkRemoveFromPlaylist: '从播放列表移除',
     bulkClear: '取消选择',
     updaterAvailable: '有可用更新',
-    updaterVersion: 'v{{version}} 已就绪',
-    updaterInstall: '安装并重启',
-    updaterDownloading: '下载中…',
-    updaterInstalling: '安装中…',
-    updaterDownload: '从 GitHub 下载',
-    updaterExperimentalHint: '自动更新功能仍在开发中',
+    updaterVersion: 'v{{version}} 已发布',
+    updaterWebsite: '官网',
   },
   settings: {
     title: '设置',