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/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>