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})

steward/templates/auth/setup.html

@@ -18,6 +18,17 @@
                 <label>Password</label>
                 <input type="password" name="password" required>
             </div>
+            <div class="form-group">
+                <label>Steward URL
+                    <span style="color:var(--text-muted);font-weight:normal;font-size:0.78rem;">(how hosts & browsers reach this server)</span>
+                </label>
+                <input type="text" name="public_base_url" value="{{ default_url or '' }}"
+                       placeholder="https://steward.example.com">
+                <span style="color:var(--text-muted);font-size:0.78rem;">
+                    Used for agent installs, share links, and alert links. Pre-filled from your current
+                    address — change it if hosts reach Steward by a different name. Editable later in Settings → General.
+                </span>
+            </div>
             <button type="submit" class="btn">Create Admin Account</button>
         </form>
     </div>