* 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)
Frank Stellmacher