feat(ui): add setup guidance when example images path is not configured

When users try to import custom example images without configuring the
download location, show a helpful guidance interface instead of failing
silently or showing an error after the fact.

Changes:
- ShowcaseView.js: Check if example_images_path is configured before
  showing import interface; display setup guidance with open settings button
- showcase.css: Add styles for the setup guidance state
- locales: Add translation keys for all 10 supported languages

Clicking 'Open Settings' will:
1. Open the settings modal
2. Scroll to the Example Images section
3. Highlight the section with a brief animation
4. Focus the input field

Fixes #785

locales/zh-TW.json

@@ -1165,7 +1165,11 @@
         "exampleImages": {
             "opened": "範例圖片資料夾已開啟",
             "openingFolder": "正在開啟範例圖片資料夾",
-            "failedToOpen": "開啟範例圖片資料夾失敗"
+            "failedToOpen": "開啟範例圖片資料夾失敗",
+            "setupRequired": "範例圖片儲存",
+            "setupDescription": "要新增自訂範例圖片，您需要先設定下載位置。",
+            "setupUsage": "此路徑用於儲存下載的範例圖片和自訂圖片。",
+            "openSettings": "開啟設定"
         }
     },
     "help": {