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/en/settings.ts

@@ -390,7 +390,7 @@ export const settings = {
   searchNoResults: 'No settings match your search.',
   integrationsPrivacyTitle: 'Privacy notice',
   integrationsPrivacyBody: 'All integrations on this tab are <strong>opt-in</strong> and send data to external services or to your Navidrome server when enabled. Last.fm receives your listening history, Discord shows the currently playing track in your profile, Bandsintown is queried per artist to fetch tour dates, and the Now-Playing share publishes your current track to other users of your Navidrome server. If you don\'t want any of this, simply leave the corresponding section disabled.',
-  homeCustomizerTitle: 'Home Page',
+  homeCustomizerTitle: 'Mainstage',
   queueModeTitle: 'Queue Display Mode',
   queueModeQueueSub: 'Show only upcoming tracks. The current track stays in the header and leaves the list once it has played.',
   queueModePlaylistSub: 'Keep the whole queue in the list with the current track highlighted at the top; played tracks stay.',