feat(theme): persist user theme preferences (#2756)

- Migrate theme persistence from `sessionStorage` to `localStorage`
- Rename theme HTML attribute to `data-bs-theme` for Bootstrap compatibility
- Trim and compile CSS according to the chosen theme mode

_data/locales/nl-NL.yml

@@ -49,6 +49,11 @@ notification:
   update_found: Nieuwe versie van inhoud beschikbaar.
   update: Update
 
+theme:
+  light: Licht
+  dark: Donker
+  system: Systeem
+
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