feat: Stellar transaction signing confirmation screen

[wumibals]

Summary

Updates the transaction confirm screen with a clear summary display, prominent fee display, confirmation checkbox, spinner during submission, and cancel returning without signing.

Changes

• Display clear summary: 'You are sending X XLM to GXXX...XXXX'
• Show network fee prominently with warning color when high
• 'Confirm and Sign' button (disabled until checkbox checked)
• 'Cancel' returns to previous screen without signing via router.back()
• ActivityIndicator spinner shown while transaction is being submitted

Acceptance Criteria

• All transaction details are legible
• Fee is clearly shown before signing
• Loading state shown during submission
• Cancel returns to the previous screen without signing

closes #268

[vercel]

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