认证页 axe a11y 审计+修复：7 处布局级真实违规全修，复扫验证 dashboard/admin/system 归零；基准 total_issues 保持 8 不变（纯质量收益）

Result: {"status":"keep","total_issues":8,"eslint_errors":0,"eslint_problems":0,"eslint_warnings":0,"golint_canonicalheader":0,"golint_errname":0,"golint_errorlint":1,"golint_exhaustive":0,"golint_forcetypeassert":0,"golint_gosec":0,"golint_intrange":0,"golint_modernize":3,"golint_nilnil":3,"golint_perfsprint":0,"golint_prealloc":0,"golint_recvcheck":1,"golint_test_testifylint":0,"golint_test_thelper":0,"golint_test_total":0,"golint_test_usetesting":0,"golint_total":8,"golint_usestdlibvars":0,"golint_vetx_total":0,"golint_wastedassign":0,"measure_s":93,"tsc_errors":0,"vitest_failed":0,"vitest_total":126}

frontend/components/layout/sidebar.tsx

@@ -197,12 +197,15 @@ export function AppSidebar({ ...props }: React.ComponentProps<typeof Sidebar>) {
       <Sidebar
         collapsible='icon'
         {...props}
+        role='navigation'
+        aria-label={t('mainNavigation')}
         className='px-2 relative border-r border-border/40 group-data-[collapsible=icon]'
       >
         <Button
           onClick={toggleSidebar}
           variant='ghost'
           size='icon'
+          aria-label={t('toggleSidebar')}
           className='absolute top-1/2 -right-6 w-2 h-4 text-muted-foreground hover:bg-background hidden md:flex'
         >
           {state === 'expanded' ? (