feat(welcome): show first-run welcome dialog on fresh install

Introduce a one-time welcome dialog that explains CC Switch's workflow
to new users: how their existing config is preserved as a "default"
provider and how the bundled "Official" preset enables one-click revert.
Upgrade users are excluded by checking is_providers_empty() at startup
and never see the dialog.

Persistence follows the existing *_confirmed convention in AppSettings
(proxy/usage/stream_check/failover), stored in settings.json. The field
is only written when the user explicitly clicks the confirm button,
keeping its semantics strictly about user acknowledgement.

Also adds two reusable DAO helpers:
- Database::is_providers_empty for fresh-install detection, using
  EXISTS(SELECT 1) for a short-circuit query.
- Database::get_bool_flag accepting "true" | "1", with
  init_default_official_providers migrated to use it.

Dialog copy in zh/en/ja uses conditional phrasing so it stays
accurate whether or not existing live config was found.

src/components/FirstRunNoticeDialog.tsx

@@ -0,0 +1,67 @@
+import { useTranslation } from "react-i18next";
+import { useQueryClient } from "@tanstack/react-query";
+import { Sparkles } from "lucide-react";
+import {
+  Dialog,
+  DialogContent,
+  DialogDescription,
+  DialogFooter,
+  DialogHeader,
+  DialogTitle,
+} from "@/components/ui/dialog";
+import { Button } from "@/components/ui/button";
+import { useSettingsQuery } from "@/lib/query";
+import { settingsApi } from "@/lib/api";
+
+/** 首次运行欢迎提示：仅当后端启动阶段保留 firstRunNoticeConfirmed 为空时弹出。 */
+export function FirstRunNoticeDialog() {
+  const { t } = useTranslation();
+  const queryClient = useQueryClient();
+  const { data: settings } = useSettingsQuery();
+
+  // 后端启动时已经决定好要不要弹：条件不满足的话字段会立即被写成 true，
+  // 所以前端这里只需要判空即可——完全对齐 streamCheckConfirmed 等既有 flag 的模式。
+  const isOpen = settings != null && settings.firstRunNoticeConfirmed !== true;
+
+  const handleAcknowledge = async () => {
+    if (!settings) return;
+    try {
+      const { webdavSync: _, ...rest } = settings;
+      await settingsApi.save({ ...rest, firstRunNoticeConfirmed: true });
+      await queryClient.invalidateQueries({ queryKey: ["settings"] });
+    } catch (error) {
+      console.error("Failed to save firstRunNoticeConfirmed:", error);
+    }
+  };
+
+  return (
+    <Dialog
+      open={isOpen}
+      onOpenChange={(open) => {
+        if (!open) void handleAcknowledge();
+      }}
+    >
+      <DialogContent className="max-w-md" zIndex="top">
+        <DialogHeader>
+          <DialogTitle className="flex items-center gap-2">
+            <Sparkles className="h-5 w-5 text-blue-500" />
+            {t("firstRunNotice.title")}
+          </DialogTitle>
+        </DialogHeader>
+        <div className="space-y-3 px-6 py-5">
+          <DialogDescription className="whitespace-pre-line leading-relaxed">
+            {t("firstRunNotice.bodyDefault")}
+          </DialogDescription>
+          <DialogDescription className="whitespace-pre-line leading-relaxed">
+            {t("firstRunNotice.bodyOfficial")}
+          </DialogDescription>
+        </div>
+        <DialogFooter>
+          <Button onClick={handleAcknowledge}>
+            {t("firstRunNotice.confirm")}
+          </Button>
+        </DialogFooter>
+      </DialogContent>
+    </Dialog>
+  );
+}