feat(auth): capture Steward URL during first-run admin setup (OOBE)

A fresh install had no prompt for general.public_base_url, so first-run agent
installs/share links silently used the request Host header. Add a "Steward URL"
field to the first-run /setup page (create-admin), pre-filled with the current
address, with help text. setup_post saves general.public_base_url and applies
it to app.config immediately (no restart). Editable later in Settings → General.

Scribe #896.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/auth/routes.py

@@ -184,7 +184,10 @@ async def setup():
     from quart import current_app
     if await get_user_count(current_app) > 0:
         return redirect(url_for("auth.login"))
-    return await render_template("auth/setup.html")
+    # Pre-fill the public URL with however the operator reached this page — the
+    # common case is correct, and it's what agent installs / share links use.
+    return await render_template(
+        "auth/setup.html", default_url=request.host_url.rstrip("/"))
 
 
 @auth_bp.post("/setup")
@@ -201,9 +204,16 @@ async def setup_post():
         return await render_template("auth/setup.html", error="All fields required"), 400
     pw_hash = bcrypt.hashpw(password, bcrypt.gensalt()).decode()
     user = User(username=username, email=email, password_hash=pw_hash, role=UserRole.admin)
+    steward_url = (form.get("public_base_url", "") or "").strip().rstrip("/")
     async with current_app.db_sessionmaker() as db:
         async with db.begin():
             db.add(user)
+            if steward_url:
+                from steward.core.settings import set_setting
+                await set_setting(db, "general.public_base_url", steward_url)
+    if steward_url:
+        # Apply immediately so agent installs / share links use it without a restart.
+        current_app.config["PUBLIC_BASE_URL"] = steward_url
     login_user(user)
     await log_audit(current_app, user.id, user.username, "auth.setup",
                     detail={"username": username})