feat: add icons to provider dialog action buttons

Add visual indicators to the Add and Save buttons in provider dialogs
for improved UX and consistency with other dialogs.

Changes:
- AddProviderDialog: Add Plus icon to "Add" button
- EditProviderDialog: Add Save icon to "Save" button

Both dialogs now have clear visual affordances for primary actions,
matching the icon usage pattern in other modal dialogs throughout
the application.

src/components/providers/AddProviderDialog.tsx

@@ -1,5 +1,6 @@
 import { useCallback } from "react";
 import { useTranslation } from "react-i18next";
+import { Plus } from "lucide-react";
 import {
   Dialog,
   DialogContent,
@@ -164,6 +165,7 @@ export function AddProviderDialog({
             {t("common.cancel", { defaultValue: "取消" })}
           </Button>
           <Button type="submit" form="provider-form">
+            <Plus className="h-4 w-4" />
             {t("common.add", { defaultValue: "添加" })}
           </Button>
         </DialogFooter>

src/components/providers/EditProviderDialog.tsx

@@ -1,5 +1,6 @@
 import { useCallback } from "react";
 import { useTranslation } from "react-i18next";
+import { Save } from "lucide-react";
 import {
   Dialog,
   DialogContent,
@@ -94,6 +95,7 @@ export function EditProviderDialog({
             {t("common.cancel", { defaultValue: "取消" })}
           </Button>
           <Button type="submit" form="provider-form">
+            <Save className="h-4 w-4" />
             {t("common.save", { defaultValue: "保存" })}
           </Button>
         </DialogFooter>