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

templates/setup.templ

@@ -49,6 +49,19 @@ templ Setup() {
 					<p class="mb-6" style="color: var(--text-secondary)">This first account will have full admin privileges.</p>
 
 					<form @submit.prevent="submitAdmin()">
+						<div class="mb-6 p-4 rounded-lg border" style="background-color: var(--bg-primary); border-color: var(--border)">
+							<label class="block text-sm font-medium mb-1" style="color: var(--text-secondary)">Server URL (Base URL)</label>
+							<input
+								type="url"
+								x-model="admin.baseUrl"
+								placeholder="https://books.example.com"
+								class="w-full px-3 py-2 border rounded mt-2"
+								style="background-color: var(--bg-secondary); color: var(--text-primary); border-color: var(--border)"
+								required
+							/>
+							<p class="text-sm mt-2" style="color: var(--text-secondary)">The public URL of your Bookhoard instance. Used for device sync, OPDS feed, and API endpoints.</p>
+						</div>
+
 						<div class="mb-4">
 							<label class="block text-sm font-medium mb-1" style="color: var(--text-secondary)">Email</label>
 							<input
@@ -289,6 +302,10 @@ templ Setup() {
 
 					<!-- Summary -->
 					<div class="space-y-3 mb-6">
+						<div class="p-4 rounded-lg border" style="background-color: var(--bg-primary); border-color: var(--border)">
+							<p class="text-sm font-medium mb-1" style="color: var(--text-secondary)">Server URL</p>
+							<p style="color: var(--text-primary)" x-text="admin.baseUrl"></p>
+						</div>
 						<div class="p-4 rounded-lg border" style="background-color: var(--bg-primary); border-color: var(--border)">
 							<p class="text-sm font-medium mb-1" style="color: var(--text-secondary)">Administrator</p>
 							<p style="color: var(--text-primary)" x-text="admin.username + ' (' + admin.email + ')'"></p>