fix(library): empty-state on Mainstage, Albums, New Releases, Random Albums (#750)

* fix(library): show empty-state on Mainstage, Albums, New Releases, Random Albums

When the active library has no albums, Mainstage and the three album-list
pages rendered a fully blank page — every rail and grid was empty but
nothing told the user why. Add a shared `common.libraryEmpty` message
(all 9 locales) and render it in place of the empty grid:

* Home: when no rail has any albums after loading.
* Albums: when no albums and no filter (genre/year/starred/comp) is
  active — filtered "no matches" is a separate state and stays as-is.
* New Releases: when no albums and no genre filter is active.
* Random Albums: when no albums after loading.

Pages that already had a dedicated empty-state (Artists, Genres,
Composers, Playlists, Favorites, MostPlayed, LosslessAlbums,
LabelAlbums, InternetRadio) are left alone — their wording is
intentional and per-page.

* docs(changelog): note empty-library states on Mainstage / album lists (#750)

src/locales/de/common.ts

@@ -63,4 +63,5 @@ export const common = {
   more: 'mehr',
   yearRange: 'Jahresbereich',
   clearAll: 'Alles zurücksetzen',
+  libraryEmpty: 'Deine Bibliothek ist leer.',
 };