fix(mainstage): rename Home Page setting, start-page fallback + empty state (#975)

* fix(mainstage): rename Home Page setting, add start-page fallback + empty state

- Rename "Home Page" personalisation section to "Mainstage" across 9 locales
  so the heading matches the sidebar entry; add "mainstage" search keyword.
- Index route "/" now redirects to the first visible library item when the
  Mainstage sidebar entry is hidden, instead of stranding the app on a blank
  page. New pure resolveStartRoute() mirrors sidebar order + nav-mode gating.
- Show a guided empty state on Mainstage when every section is toggled off,
  with a CTA into Settings -> Personalisation.
- Unit tests for resolveStartRoute.

* docs(changelog): Mainstage rename, start-page fallback + empty state (#975)

src/locales/zh/settings.ts

@@ -322,7 +322,7 @@ export const settings = {
   aboutMaintainersLabel: '维护者',
   integrationsPrivacyTitle: '隐私说明',
   integrationsPrivacyBody: '此标签页中的所有集成均为 <strong>选择加入</strong>，启用后会向外部服务或您的 Navidrome 服务器发送数据。Last.fm 接收您的收听历史，Discord 在您的个人资料中显示正在播放的曲目，Bandsintown 会按艺人查询以获取巡演日期，"正在播放" 分享会向您 Navidrome 服务器的其他用户公开您当前的曲目。如果您不需要这些功能，只需将相应部分保持停用即可。',
-  homeCustomizerTitle: '首页',
+  homeCustomizerTitle: '主舞台',
   queueModeTitle: '队列显示模式',
   queueModeQueueSub: '仅显示即将播放的曲目。当前曲目保留在标题栏中，播放完毕后从列表移除。',
   queueModePlaylistSub: '在列表中保留整个队列，当前曲目在顶部高亮显示；已播放的曲目保留。',