feat: add Clear button to Settings alongside Randomize

[samdaw13]

Summary

• Adds a Clear button to the Enigma machine Settings screen, placed on the same row as the existing Randomize button
• Pressing Clear resets all rotor selections and plugboard cables to defaults
• Both buttons share equal width via flex: 1 in a buttonRow layout
• Adds CLEAR_SETTINGS label and CLEAR_BUTTON testID constants
• Adds a test verifying the Clear button resets rotor state

Test plan

• Press Clear — all three rotor slots are deselected and plugboard cables are removed
• Press Randomize — rotors and plugboard are populated with random values (existing behaviour unchanged)
• Encrypt a message button remains disabled after pressing Clear (no rotors selected)
• Both buttons display side-by-side at equal width on the Settings screen
• npm test passes
• npm run lint passes

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