feat(i18n): Dutch locale — 8th bundled language

Ports PR #561 by @pbaart into the live gettext system (the PR
translated the retired Svelte catalog): full nl.po (1481 msgids, 13
plurals, msgfmt-clean, zero untranslated), 262 strings taken directly
from the contribution via the legacy-English bridge and the rest
translated using the contributor's terminology as the style guide
(nummer/afspeellijst/wachtrij/bibliotheek, informal je). qbz-i18n
registration (2-form plural, resolve_auto nl_NL->nl, 21/21 tests) and
the language dropdown at index 8 (Nederlands) across ui_prefs,
reseed_i18n_labels and state.slint — same shape as the ru/ja commits.

Dutch translation credit: pbaart (PR #561).

cc

crates/qbz-ui/ui/state.slint

@@ -2950,7 +2950,7 @@ export global AppearanceState {
     // that lived here were superseded by the working interface-size presets
     // (`ui-scale-options` / `ui-scale-index` in the INTERFACE SIZE block
     // further down, wired to SLINT_SCALE_FACTOR in main.rs).
-    in-out property <[string]> languages: [@tr("Auto"), "English", "Español", "Français", "Deutsch", "Português", "Русский", "日本語"];
+    in-out property <[string]> languages: [@tr("Auto"), "English", "Español", "Français", "Deutsch", "Português", "Русский", "日本語", "Nederlands"];
     in-out property <int> language-index: 0;
     in-out property <[string]> fonts: ["LINE Seed JP", "Montserrat", "Noto Sans", "Source Sans 3", "System"];
     in-out property <int> font-index: 0;