fix(desktop): clarify identity key button when key exists

[Chessing234]

Summary

• backing out of the backup step and clicking the CTA again reuses the existing key
• the button still said "Create a new identity key", which is misleading for a security-sensitive action
• when a key is already loaded, label it "Continue with existing identity key"

Closes #2318

Test plan

• first-run: button still says "Create a new identity key"
• create key → backup → back: button says "Continue with existing identity key" and shows the same key

Made with Cursor

[Chessing234]

button label switches to "continue with existing identity key" once a key is already loaded.