fix(a11y): add aria-live to password strength, role=meter to signal bar

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/monitoring/SignalBar.tsx

@@ -46,6 +46,11 @@ export function SignalBar({ signal, label }: SignalBarProps) {
           <div
             className={cn('h-full rounded-full transition-all', barColor)}
             style={{ width: `${pct}%` }}
+            role="meter"
+            aria-label={label || 'Signal strength'}
+            aria-valuenow={signal ?? 0}
+            aria-valuemin={-100}
+            aria-valuemax={0}
           />
         </div>
         {/* dBm value */}