fix: hardcoded color, missing async cancellation, a11y gaps, any types

SealingKeysView: replace hardcoded #666 with var(--mui-palette-text-secondary)
SealedSecretDetail:
- Add cancelled flag to canDecryptSecrets useEffect
- Add aria-label to close button, decrypt buttons, delete dialog
- Replace any types in SimpleTable column getters with { key, value }

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/SealingKeysView.tsx

@@ -189,7 +189,7 @@ export function SealingKeysView() {
                   return (
                     <Box sx={{ display: 'flex', alignItems: 'center', gap: 1 }}>
                       <span>{expiryDate}</span>
-                      <span style={{ color: '#666', fontSize: '0.9em' }}>
+                      <span style={{ color: 'var(--mui-palette-text-secondary, #666)', fontSize: '0.9em' }}>
                         ({certInfo.daysUntilExpiry} days)
                       </span>
                     </Box>