Enhance ConnectionPlaceholder component to handle missing device IP with localized messages; update DeviceContext to manage IP state; improve CSS for touch actions in various components; update locale files for English and Chinese with new connection prompts.

client/src/locales/data-en.json

@@ -1,4 +1,12 @@
 {
+  "connect": {
+    "ipMissingTitle": "Device IP not specified",
+    "ipMissingDesc": "Add the device IP in the address bar before opening this page, for example:",
+    "connecting": "Connecting to {ip}…",
+    "failedTitle": "Connection failed",
+    "failedDesc": "Check the device IP and your network connection.",
+    "failedHint": "Specify the device in the URL, e.g. ?ip=10.0.0.119"
+  },
   "home": {
     "audioSet": "Audio setting",
     "systemSet": "System",

client/src/locales/data-zh-HK.json

@@ -1,4 +1,12 @@
 {
+  "connect": {
+    "ipMissingTitle": "未指定裝置 IP",
+    "ipMissingDesc": "請在網址列加入裝置 IP 參數後再開啟本頁，例如：",
+    "connecting": "正在連線 {ip}…",
+    "failedTitle": "連線失敗",
+    "failedDesc": "請檢查裝置 IP 與網路連線。",
+    "failedHint": "透過網址列指定裝置，例如 ?ip=10.0.0.119"
+  },
   "home": {
     "audioSet": "音訊設定",
     "systemSet": "系統設定",

client/src/locales/data-zh.json

@@ -1,4 +1,12 @@
 {
+  "connect": {
+    "ipMissingTitle": "未指定设备 IP",
+    "ipMissingDesc": "请在地址栏添加设备 IP 参数后再打开本页，例如：",
+    "connecting": "正在连接 {ip}…",
+    "failedTitle": "连接失败",
+    "failedDesc": "请检查设备 IP 与网络连接。",
+    "failedHint": "通过地址栏指定设备，例如 ?ip=10.0.0.119"
+  },
   "home": {
     "audioSet": "音频设置",
     "systemSet": "系统设置",