feat(setup): collect server base URL during first-run wizard

Add a Server URL field to the admin-account step of the setup wizard so
the public base URL (used for device sync, OPDS feed, and API endpoints)
is configured up front instead of requiring a later visit to admin
settings.

- setup.templ: base URL input on the admin step plus a summary entry
- setup.ts: default baseUrl to window.location.origin and persist it via
  PUT /system/config ({ base_url }) right after the admin account is
  created, with a non-fatal warning if the save fails

web/src/setup.ts

@@ -36,14 +36,13 @@ function initSetup(): void {
   if (savedTheme && savedTheme !== "tokyo-night") {
     applyTheme(savedTheme);
   }
-}
-
-Alpine.data("setupWizard", () => ({
+}Alpine.data("setupWizard", () => ({
   currentStep: 1 as number,
   loading: false as boolean,
   errorMsg: "" as string,
 
   admin: {
+    baseUrl: "",
     email: "",
     username: "",
     password: "",
@@ -71,6 +70,7 @@ Alpine.data("setupWizard", () => ({
 
   init() {
     initSetup();
+    this.admin.baseUrl = window.location.origin;
     setTimeout(() => {
       initPasswordValidation();
     }, 100);
@@ -128,6 +128,8 @@ Alpine.data("setupWizard", () => ({
       setToken(data.token || data.access_token);
       setRefreshToken(data.refresh_token);
 
+      await this.saveBaseUrl();
+
       showToast("Admin account created successfully!", "success");
       this.currentStep = 2;
     } catch (err) {
@@ -137,6 +139,17 @@ Alpine.data("setupWizard", () => ({
     }
   },
 
+  async saveBaseUrl() {
+    if (!this.admin.baseUrl) return;
+    try {
+      await handleVoidResponse(
+        await apiPut("/system/config", { base_url: this.admin.baseUrl })
+      );
+    } catch {
+      showToast("Warning: could not save server URL. You can set it later in Admin Settings.", "warning");
+    }
+  },
+
   async addLibrary() {
     this.clearError();
     this.loading = true;