Add reusable SSH identities

[R055LE]

Summary

• Add reusable Room-backed SSH identities with a username, authentication choice, and optional imported-key reference.
• Migrate each existing host profile to a separate unconfigured identity without inferring credentials or key associations.
• Let profiles prefer and reuse identities; confirm identity deletion, then unlink profiles while retaining saved keys.
• Keep passwords and key passphrases session-only, and require an unlinked saved profile to choose an identity before connecting.

Validation

• ./gradlew test lint assembleDebug
• 36 focused Android tests on the API 35 emulator covering migrations, identity storage, profile selection, and deletion confirmation.

Closes #45