refactor(ui): unify search bar placeholder to i18n key header.search.placeholder

- Replace page-specific header.search.placeholders.* keys with a single
  header.search.placeholder key (value: "Search", no ellipsis)
- Keep header.search.notAvailable for the statistics page
- Remove unused placeholder/placeholders/notAvailable entries from all
  10 locale files; preserve options and searchIn keys
- Update Jinja template and JS header to use the new unified key

locales/ja.json

@@ -202,13 +202,7 @@
             "statistics": "統計"
         },
         "search": {
-            "placeholder": "検索...",
-            "placeholders": {
-                "loras": "LoRAを検索...",
-                "recipes": "レシピを検索...",
-                "checkpoints": "checkpointを検索...",
-                "embeddings": "embeddingを検索..."
-            },
+            "placeholder": "検索",
             "options": "検索オプション",
             "searchIn": "検索対象：",
             "notAvailable": "統計ページでは検索は利用できません",