feat: add CA-signed certificate authentication for SSH credentials

Support OpenSSH certificate-based authentication (-cert.pub files) in
the Credentials manager. When a CA-signed certificate is stored alongside
a private key, Termix uses it during SSH connection establishment so that
servers relying on certificate-based authorization work out of the box.

Changes:
- db/schema.ts: add cert_public_key column to ssh_credentials table
- db/index.ts: auto-migration via addColumnIfNotExists
- routes/credentials.ts: expose certPublicKey in create/update/get endpoints
- ssh/auth-manager.ts: include certPublicKey in ResolvedCredentials
- ssh/host-resolver.ts: propagate certPublicKey when resolving credentials
- ssh/opkssh-cert-auth.ts: refactor shared logic into _applyCertToConnection;
  export new setupCACertAuth() with optional passphrase support
- ssh/terminal.ts: call setupCACertAuth() when a certificate is present
- utils/ssh-key-utils.ts: detect all OpenSSH cert types in public key parser
- types/index.ts: add certPublicKey to Credential, CredentialBackend,
  CredentialData interfaces
- CredentialAuthenticationTab.tsx: new CA Certificate section with file
  upload, paste editor and automatic cert-type badge
- CredentialEditor.tsx: certPublicKey wired into form schema and submit
- CredentialViewer.tsx: show certificate status in security tab
- locales/en.json: add i18n strings for new UI elements

src/backend/database/db/index.ts

@@ -710,6 +710,8 @@ const migrateSchema = () => {
   addColumnIfNotExists("ssh_credentials", "public_key", "TEXT");
   addColumnIfNotExists("ssh_credentials", "detected_key_type", "TEXT");
 
+  addColumnIfNotExists("ssh_credentials", "cert_public_key", "TEXT");
+
   addColumnIfNotExists("ssh_credentials", "system_password", "TEXT");
   addColumnIfNotExists("ssh_credentials", "system_key", "TEXT");
   addColumnIfNotExists("ssh_credentials", "system_key_password", "TEXT");

src/backend/database/db/schema.ts

@@ -256,6 +256,8 @@ export const sshCredentials = sqliteTable("ssh_credentials", {
   keyType: text("key_type"),
   detectedKeyType: text("detected_key_type"),
 
+  certPublicKey: text("cert_public_key", { length: 8192 }),
+
   systemPassword: text("system_password"),
   systemKey: text("system_key", { length: 16384 }),
   systemKeyPassword: text("system_key_password"),