feat(models): add group-by-model option to collapse multiple versions into one card

Adds a 'Group by Model' toggle in Layout Settings. When enabled, only the
latest version (highest civitai.id) of each Civitai model is shown as a
single card — older versions sharing the same modelId are hidden.

Backend dedup runs in BaseModelService.get_paginated_data() before
filtering/pagination, ensuring correct paginated results. The setting
is persisted via the existing settings pipeline and passed as a query
parameter to the listing endpoint.

Includes:
- Backend: dedup logic, route param parsing, settings default
- Frontend: API param, SettingsManager wiring, toggle UI
- i18n: translations for all 10 locales
- Tests: unit test covering dedup on/off and standalone items

locales/zh-CN.json

@@ -430,6 +430,8 @@
             "help": "启用后，如果下载历史服务记录显示该版本已下载，LoRA Manager 将跳过下载该模型版本。适用于所有下载流程。"
         },
         "layoutSettings": {
+            "groupByModel": "按模型分组",
+            "groupByModelHelp": "开启后，每个 Civitai 模型仅显示最新版本的单张卡片，旧版本将被隐藏。",
             "displayDensity": "显示密度",
             "displayDensityOptions": {
                 "default": "默认",