feat: add ESC key navigation support in settings menus

[bvdr]

Summary

Implements intuitive ESC key navigation throughout the settings menu system for improved user experience.

Changes

• ✅ ESC in top-level settings menu exits settings
• ✅ ESC in nested menus returns to parent menu
• ✅ Settings menu loops after completing actions
• ✅ Add explicit "← Exit Settings" and "← Back to Settings" options
• ✅ Modified updateCredentials() to return navigation signals

Menu Navigation Flow

1. Main Settings Menu → ESC exits / "← Exit Settings" option
2. Credential Update Menu → ESC returns to settings / "← Back to Settings" option
3. Provider Selection → ESC returns to settings
4. Model Selection → ESC returns to settings
5. After any action → Automatically returns to settings menu

Technical Implementation

• Wrapped showSettings() in while(true) loop for continuous navigation
• All isCancel() checks now use continue to return to parent menu
• Added return value system ('back', 'continue') for submenu navigation
• Global preferences object updates in real-time

Files Modified

• index.js - Settings menu system with ESC navigation (+141 lines, -112 lines refactored)

Testing

• ESC exits top-level settings menu
• ESC returns from credential update submenu
• ESC returns from provider/model selection
• Explicit back/exit options work
• Settings persist across menu loops

Fixes #24