feat(core): sync framework security hardening and accessibility improvements

- add util.Go with panic recovery for background goroutines
- add util.EscapeLike and explicit ESCAPE clause for SQL LIKE queries
- add DummyCheckPassword and subtle.ConstantTimeCompare against timing attacks
- enforce session ID rotation upon login/oauth callback to prevent session fixation
- add sliding window login failure rate limiting and oauth state rate limiting
- fix redis client capture race in pubsub listeners and wait on stop channel
- adjust global --primary to oklch(51.1% 0.262 276.966) for WCAG AA contrast
- fix semantic heading levels and missing aria-labels across UI components
- document security, concurrency, and a11y standards in AGENTS.md

frontend/components/common/user/file-manager.tsx

@@ -238,7 +238,7 @@ export function UserFileManager() {
               <Upload className='size-10' />
             </div>
             <div className='space-y-1'>
-              <h3 className='font-semibold text-sm'>{t('noFiles')}</h3>
+              <p className='font-semibold text-sm'>{t('noFiles')}</p>
               <p className='text-xs text-muted-foreground max-w-xs'>
                 {debouncedKeyword ? t('noMatchingFiles') : t('uploadFirstFile')}
               </p>