fix: add optional gradient toggle to primary button

[jvsena42]

Fixes #692

This PR adds an optional enableGradient parameter to PrimaryButton to allow disabling the gradient background for selected word buttons in the mnemonic confirmation flow.

Description

• Adds enableGradient: Boolean = true parameter to PrimaryButton
• Adds corresponding parameter to primaryButtonStyle modifier
• When disabled, uses solid color instead of gradient while preserving shadow and border highlight
• Updates mnemonic word buttons to use solid background

Preview

Screen_recording_20260122_184223.webm

QA Notes

1. Navigate to Settings > Backups > Back Up Now
2. Complete the mnemonic display step to reach the confirmation screen
3. Select several words from the word bank
4. Verify selected words show a solid background (no gradient)
5. Verify other PrimaryButton instances (Continue button) still show the gradient

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[ovitrif]

utAck

LGTM 🥇