fix: center sidebar footer divider and correct Apps arrow

Use equal-width Apps and Settings buttons so the divider stays centered. Increase divider visibility and point the Apps arrow upward to match the menu opening direction.

Release note: Make the sidebar footer divider clearer and centered, and align the Apps arrow with its upward-opening menu.

docs/features/application-tool-navigation/technical-design.md

@@ -179,7 +179,7 @@ Main 继续校验可信 sender、Zod 输入，复用 `ApplicationSettingsStore`
   <button className="nav-item" onClick={toggleApplicationMenu} aria-haspopup="menu" aria-expanded={applicationMenuOpen} ...>
     <Grid2X2 aria-hidden="true" />
     <span>{t('applications.menuLabel')}</span>
-    <ChevronDown aria-hidden="true" />
+    <ChevronUp aria-hidden="true" />
   </button>
   <div className="sidebar-footer__divider" aria-hidden="true" />
   <button className="nav-item" aria-label={t('navigation.settings')} ...>