feat(ui): auto-theme settings UI + i18n

Repoint the existing auto-theme Settings scaffold from the "System" theme to
the new dynamic "Auto (dynamic)" option:

- state.slint: add theme-is-auto (distinct from theme-is-system, which stays
  the native OS-palette follow).
- AppearanceSettings.slint: gate the Source dropdown, image picker, detected-DE
  hint and Regenerate button on theme-is-auto, and drop the placeholder
  `unsupported` flag now that the rows are functional.
- translations: add "Auto theme generation failed" to all 7 catalogs and give
  the es "Wallpaper Sync" string a proper Mexican-neutral translation.

cc

crates/qbz-ui/translations/de/LC_MESSAGES/qbz-ui.po

@@ -4370,3 +4370,6 @@ msgstr "Audioausgabefehler"
 
 msgid "Direct ALSA device access is blocked inside Flatpak/Snap — switch the audio backend to PipeWire or System Default"
 msgstr "Direkter ALSA-Gerätezugriff ist in Flatpak/Snap blockiert — Audio-Backend auf PipeWire oder Systemstandard umstellen"
+
+msgid "Auto theme generation failed"
+msgstr "Automatische Themenerstellung fehlgeschlagen"

crates/qbz-ui/translations/en/LC_MESSAGES/qbz-ui.po

@@ -136,3 +136,6 @@ msgstr "No labels in your Library yet."
 
 msgid "Library Albums"
 msgstr "Library Albums"
+
+msgid "Auto theme generation failed"
+msgstr "Auto theme generation failed"

crates/qbz-ui/translations/es/LC_MESSAGES/qbz-ui.po

@@ -12,7 +12,7 @@ msgid "System Colors"
 msgstr "Colores del Sistema"
 
 msgid "Wallpaper Sync"
-msgstr "Wallpaper Sync"
+msgstr "Sincronizar con el fondo"
 
 msgid "Custom Image"
 msgstr "Imagen Personalizada"
@@ -4400,3 +4400,6 @@ msgstr "Error de salida de audio"
 
 msgid "Direct ALSA device access is blocked inside Flatpak/Snap — switch the audio backend to PipeWire or System Default"
 msgstr "El acceso directo a dispositivos ALSA está bloqueado dentro de Flatpak/Snap — cambia el backend de audio a PipeWire o al predeterminado del sistema"
+
+msgid "Auto theme generation failed"
+msgstr "Error al generar el tema automático"

crates/qbz-ui/translations/fr/LC_MESSAGES/qbz-ui.po

@@ -4370,3 +4370,6 @@ msgstr "Erreur de sortie audio"
 
 msgid "Direct ALSA device access is blocked inside Flatpak/Snap — switch the audio backend to PipeWire or System Default"
 msgstr "L'accès direct aux périphériques ALSA est bloqué dans Flatpak/Snap — passez le backend audio sur PipeWire ou la sortie système"
+
+msgid "Auto theme generation failed"
+msgstr "Échec de la génération du thème automatique"

crates/qbz-ui/translations/ja/LC_MESSAGES/qbz-ui.po

@@ -4359,3 +4359,6 @@ msgstr "オーディオ出力エラー"
 
 msgid "Direct ALSA device access is blocked inside Flatpak/Snap — switch the audio backend to PipeWire or System Default"
 msgstr "Flatpak/Snap 内では ALSA デバイスへの直接アクセスがブロックされます — オーディオバックエンドを PipeWire またはシステム既定に切り替えてください"
+
+msgid "Auto theme generation failed"
+msgstr "自動テーマの生成に失敗しました"

crates/qbz-ui/translations/pt/LC_MESSAGES/qbz-ui.po

@@ -4370,3 +4370,6 @@ msgstr "Erro de saída de áudio"
 
 msgid "Direct ALSA device access is blocked inside Flatpak/Snap — switch the audio backend to PipeWire or System Default"
 msgstr "O acesso direto a dispositivos ALSA está bloqueado no Flatpak/Snap — mude o backend de áudio para PipeWire ou o padrão do sistema"
+
+msgid "Auto theme generation failed"
+msgstr "Falha ao gerar o tema automático"

crates/qbz-ui/translations/ru/LC_MESSAGES/qbz-ui.po

@@ -4383,3 +4383,6 @@ msgstr "Ошибка аудиовывода"
 
 msgid "Direct ALSA device access is blocked inside Flatpak/Snap — switch the audio backend to PipeWire or System Default"
 msgstr "Прямой доступ к устройствам ALSA заблокирован в Flatpak/Snap — переключите аудиобэкенд на PipeWire или системный вывод"
+
+msgid "Auto theme generation failed"
+msgstr "Не удалось сгенерировать автоматическую тему"