Add comprehensive SSH key management and validation features

- Add support for both private and public key storage
- Implement automatic SSH key type detection for all major formats (RSA, Ed25519, ECDSA, DSA)
- Add real-time key pair validation to verify private/public key correspondence
- Enhance credential editor UI with unified key input interface supporting upload/paste
- Improve file format support including extensionless files (id_rsa, id_ed25519, etc.)
- Add comprehensive fallback detection for OpenSSH format keys
- Implement debounced API calls for better UX during real-time validation
- Update database schema with backward compatibility for existing credentials
- Add API endpoints for key detection and pair validation
- Fix SSH2 module integration issues in TypeScript environment

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/backend/database/db/schema.ts

@@ -137,9 +137,12 @@ export const sshCredentials = sqliteTable("ssh_credentials", {
   authType: text("auth_type").notNull(),
   username: text("username").notNull(),
   password: text("password"),
-  key: text("key", { length: 16384 }),
+  key: text("key", { length: 16384 }), // backward compatibility
+  privateKey: text("private_key", { length: 16384 }),
+  publicKey: text("public_key", { length: 4096 }),
   keyPassword: text("key_password"),
   keyType: text("key_type"),
+  detectedKeyType: text("detected_key_type"),
   usageCount: integer("usage_count").notNull().default(0),
   lastUsed: text("last_used"),
   createdAt: text("created_at")