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"

crates/qbz-ui/ui/settings/AppearanceSettings.slint

@@ -199,9 +199,8 @@ export component AppearanceSettings inherits VerticalLayout {
         }
     }
 
-    // Auto-Theme controls — only when the "System" theme is selected.
-    if AppearanceState.theme-is-system: SettingRow {
-        unsupported: true;
+    // Auto-Theme controls — only when the "Auto (dynamic)" theme is selected.
+    if AppearanceState.theme-is-auto: SettingRow {
         label: @tr("Source");
         description: @tr("Generate a color theme from your system wallpaper or a custom image");
         QbzSelect {
@@ -214,8 +213,7 @@ export component AppearanceSettings inherits VerticalLayout {
             }
         }
     }
-    if AppearanceState.theme-is-system && AppearanceState.auto-theme-source-index == 2: SettingRow {
-        unsupported: true;
+    if AppearanceState.theme-is-auto && AppearanceState.auto-theme-source-index == 2: SettingRow {
         label: AppearanceState.auto-theme-custom-path != ""
             ? AppearanceState.auto-theme-custom-path
             : @tr("Select Image...");
@@ -226,12 +224,11 @@ export component AppearanceSettings inherits VerticalLayout {
             }
         }
     }
-    if AppearanceState.theme-is-system && AppearanceState.auto-theme-detected-de != "": SettingRow {
+    if AppearanceState.theme-is-auto && AppearanceState.auto-theme-detected-de != "": SettingRow {
         label: @tr("Detected: ") + AppearanceState.auto-theme-detected-de;
         description: @tr("Experimental: theme may not match your system exactly.");
     }
-    if AppearanceState.theme-is-system: SettingRow {
-        unsupported: true;
+    if AppearanceState.theme-is-auto: SettingRow {
         label: @tr("Regenerate");
         SecondaryButton {
             label: @tr("Regenerate");

crates/qbz-ui/ui/state.slint

@@ -2898,9 +2898,11 @@ export global AppearanceState {
 
     in-out property <bool> album-header-gradient: true;
 
-    // Auto-Theme (shown when the "System" theme is selected). Source:
-    // 0 = System Colors, 1 = Wallpaper Sync, 2 = Custom Image.
+    // True while the "System" theme is selected (native OS-palette follow).
     in-out property <bool> theme-is-system: false;
+    // Auto-Theme (shown when the appended "Auto (dynamic)" theme is selected).
+    // Source: 0 = System Colors, 1 = Wallpaper Sync, 2 = Custom Image.
+    in-out property <bool> theme-is-auto: false;
     in-out property <[string]> auto-theme-sources: [@tr("System Colors"), @tr("Wallpaper Sync"), @tr("Custom Image")];
     in-out property <int> auto-theme-source-index: 0;
     in-out property <string> auto-theme-custom-path: "";