Fix settings loading and use 2-column card layout.

Merge Flask GET/POST routes, refresh prefs on tab revisit after preload, and reorganize system settings into separate cards.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/instance/templates/display_prefs_panel.html

@@ -1,5 +1,5 @@
 {# 系统设置 · 导航显示开关 #}
-<div class="card settings-card settings-card--compact" id="display-prefs-card">
+<div class="card settings-card settings-card--standalone" id="display-prefs-card">
     <h2>导航显示</h2>
     <p class="settings-env-hint">以下开关控制顶栏导航与系统设置内区块是否显示，保存后立即生效。关键位监控、实盘下单、系统设置为固定项。</p>
     <div id="display-prefs-form" class="display-prefs-form">