Fix wallet balance + decimal formatting

[Klakurka]

Related to #492.

Description

Fixed wallet balance display formatting to show consistent decimal places across different cryptocurrencies:

• XEC Balance: Now displays exactly 2 decimal places (e.g., "1,234.56 XEC")
• BCH Balance: Now displays exactly 4 decimal places (e.g., "0.1234 BCH") - 8 felt too much but we could do that.
• Locale Support: Uses the user's browser locale for number formatting (respects international formatting preferences for thousands separators and decimal points)

This provides a more consistent and professional appearance for wallet balances while maintaining appropriate precision for each cryptocurrency type.

Test plan

1. Open /wallets.
2. View wallet with XEC balance: Verify XEC balance shows exactly 2 decimal places
3. View wallet with BCH balance: Verify BCH balance shows exactly 4 decimal places
4. Test different locales: Change browser language/locale settings and verify number formatting adapts appropriately (e.g., comma vs period for decimal separator)
5. Test edge cases:
   • Very small balances (should still show required decimal places)
   • Large balances (should show thousands separators according to locale)
   • Zero balances (should not display balance sections)

[Klakurka]

I get a weird 'hydration' error locally when I refresh the /wallets page but it also happens on master so I'm not sure what that is.