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/zh-TW.json

@@ -202,13 +202,7 @@
             "statistics": "統計"
         },
         "search": {
-            "placeholder": "搜尋...",
-            "placeholders": {
-                "loras": "搜尋 LoRA...",
-                "recipes": "搜尋配方...",
-                "checkpoints": "搜尋 checkpoint...",
-                "embeddings": "搜尋 embedding..."
-            },
+            "placeholder": "搜尋",
             "options": "搜尋選項",
             "searchIn": "搜尋範圍：",
             "notAvailable": "統計頁面無法搜尋",