fix: implement persistent SSH key storage with key generation

[michelroegl-brunner]

🐛 Fix: SSH Key Authentication Issues

Problem

• SSH key authentication was failing with 'error in libcrypto' during script execution
• Temporary key files were causing race conditions and corruption
• Confusing 'both' authentication option was causing user confusion
• No way to generate SSH keys within the application

Solution

• ✅ Persistent Key Storage: Keys are now stored in data/ssh-keys/ directory
• ✅ Key Generation: Added 'Generate Key Pair' button for automatic ED25519 key creation
• ✅ Public Key Viewing: Added 'View Public Key' button with copy-to-clipboard functionality
• ✅ Simplified Auth: Removed 'both' option, now only supports 'password' OR 'key'
• ✅ Enhanced UX: Hide manual key input when key pair is generated
• ✅ Database Migration: Added ssh_key_path and key_generated columns
• ✅ API Endpoints: New endpoints for key generation and public key retrieval

Technical Changes

• Fixed 'error in libcrypto' by eliminating temporary file race conditions
• Added persistent SSH key file management with proper permissions (0600/0644)
• Updated database schema with proper migrations
• Enhanced UI/UX with conditional rendering and success feedback
• Added comprehensive error handling and type safety

Testing

• ✅ Build compiles successfully
• ✅ All TypeScript errors resolved
• ✅ Key generation and viewing functionality working
• ✅ Copy-to-clipboard with fallback support

Resolves SSH authentication failures during script execution.