feat: add CA-signed certificate authentication for SSH credentials

[black0utdev]

Overview

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 Made

• 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

Screenshots / Demos

Checklist

• Code follows project style guidelines
• Supports mobile and desktop UI/app (if applicable)
• I have read Contributing.md
• This is not a translation request. See docs

[LukeGus]

Merged manually into dev-2.3.0 via rebase. Conflicts resolved: updated UI changes to match the 2.3.0 UI rewrite (HostManager.tsx) and moved i18n strings to the new locales path.