feat: implement certificate validation and expiry detection (Phase 2.1)

Add comprehensive certificate metadata parsing and expiry warnings.

## Changes

### Types (src/types.ts)
- Add CertificateInfo interface with validity dates, expiry status, issuer/subject, fingerprint

### Crypto Module (src/lib/crypto.ts)
- Add parseCertificateInfo() to extract certificate metadata
- Add isCertificateExpiringSoon() helper (default 30 days threshold)
- Calculate SHA-256 fingerprint, parse DN fields, compute days until expiry

### SealingKeysView (src/components/SealingKeysView.tsx)
- Display certificate expiry information in table
- Show visual indicators: Expired (red), Expiring Soon (warning), Valid (normal)
- Display days remaining for expiring certificates

### EncryptDialog (src/components/EncryptDialog.tsx)
- Add expiry warning before encryption
- Warn if certificate expired or expiring within 30 days
- Show specific expiry date in warning message

## Features

- **Certificate Parsing:** Extract all metadata from X.509 certificates
- **Expiry Detection:** Automatic detection of expired/expiring certificates
- **Visual Indicators:** Color-coded chips for expiry status
- **Proactive Warnings:** Alert users before creating secrets with expiring certs
- **SHA-256 Fingerprint:** Unique certificate identification

## Verification

- TypeScript: 0 errors
- Linting: 0 errors
- Build: Success (343.95 kB, 94.58 kB gzipped)

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

headlamp-sealed-secrets/src/types.ts

@@ -259,3 +259,25 @@ export interface EncryptionRequest {
   scope: SealedSecretScope;
   keyValues: SecretKeyValue[];
 }
+
+/**
+ * Certificate information extracted from PEM certificate
+ */
+export interface CertificateInfo {
+  /** Validity period start date */
+  validFrom: Date;
+  /** Validity period end date */
+  validTo: Date;
+  /** Whether certificate is currently expired */
+  isExpired: boolean;
+  /** Days until expiry (negative if expired) */
+  daysUntilExpiry: number;
+  /** Certificate issuer (formatted as DN string) */
+  issuer: string;
+  /** Certificate subject (formatted as DN string) */
+  subject: string;
+  /** SHA-256 fingerprint of certificate */
+  fingerprint: string;
+  /** Serial number of certificate */
+  serialNumber: string;
+}