feat(playlists): add ZIP download button to playlist detail page

Mirrors the album download UX: Download (ZIP) button in the playlist
hero header, progress bar while downloading, folder picker via
downloadModalStore (remembers last used folder). Uses the same
Subsonic /rest/download.view endpoint as album downloads — it accepts
playlist IDs just as well as album IDs.

Closes #127

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/de.ts

@@ -775,6 +775,7 @@ export const deTranslation = {
     removeCover: 'Foto entfernen',
     coverUpdated: 'Cover aktualisiert',
     metaSaved: 'Playlist aktualisiert',
+    downloadZip: 'Herunterladen (ZIP)',
   },
   radio: {
     title: 'Internetradio',

src/locales/en.ts

@@ -776,6 +776,7 @@ export const enTranslation = {
     removeCover: 'Remove photo',
     coverUpdated: 'Cover updated',
     metaSaved: 'Playlist updated',
+    downloadZip: 'Download (ZIP)',
   },
   radio: {
     title: 'Internet Radio',

src/locales/fr.ts

@@ -775,6 +775,7 @@ export const frTranslation = {
     removeCover: 'Supprimer la photo',
     coverUpdated: 'Pochette mise à jour',
     metaSaved: 'Playlist mise à jour',
+    downloadZip: 'Télécharger (ZIP)',
   },
   radio: {
     title: 'Radio Internet',

src/locales/nb.ts

@@ -774,6 +774,7 @@ export const nbTranslation = {
     removeCover: 'Fjern bilde',
     coverUpdated: 'Omslag oppdatert',
     metaSaved: 'Spillelisten er oppdatert',
+    downloadZip: 'Last ned (ZIP)',
   },
   radio: {
     title: 'Internettradio',

src/locales/nl.ts

@@ -775,6 +775,7 @@ export const nlTranslation = {
     removeCover: 'Foto verwijderen',
     coverUpdated: 'Omslag bijgewerkt',
     metaSaved: 'Playlist bijgewerkt',
+    downloadZip: 'Downloaden (ZIP)',
   },
   radio: {
     title: 'Internetradio',

src/locales/ru.ts

@@ -829,6 +829,7 @@ export const ruTranslation = {
     removeCover: 'Убрать фото',
     coverUpdated: 'Обложка обновлена',
     metaSaved: 'Плейлист сохранён',
+    downloadZip: 'Скачать (ZIP)',
   },
   radio: {
     title: 'Онлайн-радио',

src/locales/zh.ts

@@ -771,6 +771,7 @@ export const zhTranslation = {
     removeCover: '删除照片',
     coverUpdated: '封面已更新',
     metaSaved: '播放列表已更新',
+    downloadZip: '下载 (ZIP)',
   },
   radio: {
     title: '网络电台',