fix(frontend): use placeholder for IP preference select (#217)

## Summary
- Replace empty key SelectItem with proper placeholder for IP preference
dropdown
- Improves UX consistency with other select components

vite-frontend/src/pages/forward.tsx

@@ -2349,7 +2349,7 @@ export default function ForwardPage() {
 
                   <Select
                     label="限速规则"
-                    placeholder="不限速"
+                    placeholder="请选择限速规则"
                     selectedKeys={
                       selectedSpeedId !== null
                         ? [selectedSpeedId.toString()]

vite-frontend/src/pages/tunnel.tsx

@@ -1212,6 +1212,7 @@ export default function TunnelPage() {
                     <Select
                       description="当节点同时拥有IPv4和IPv6地址时，选择隧道连接使用的地址类型"
                       label="隧道连接地址偏好"
+                      placeholder="自动选择"
                       selectedKeys={[form.ipPreference || ""]}
                       variant="bordered"
                       onSelectionChange={(keys) => {
@@ -1223,7 +1224,6 @@ export default function TunnelPage() {
                         }));
                       }}
                     >
-                      <SelectItem key="">自动选择</SelectItem>
                       <SelectItem key="v4">优先IPv4</SelectItem>
                       <SelectItem key="v6">优先IPv6</SelectItem>
                     </Select>

vite-frontend/src/pages/user.tsx

@@ -1170,7 +1170,7 @@ export default function UserPage() {
                                   classNames={{
                                     trigger: "min-h-10 h-10",
                                   }}
-                                  placeholder="限速规则"
+                                  placeholder="请选择限速规则"
                                   selectedKeys={
                                     batchTunnelSelections.get(tunnel.id) !==
                                       null &&
@@ -1429,6 +1429,7 @@ export default function UserPage() {
 
                   <Select
                     label="限速规则"
+                    placeholder="请选择限速规则"
                     selectedKeys={
                       editTunnelSelectedSpeedId !== null
                         ? [editTunnelSelectedSpeedId.toString()]