Prevent entering oversized custom donation amounts

[abgolor]

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• I am following the Code Style Guidelines
• I have tested my contribution on these devices:
  • Google Pixel 8, Android 16 (Build: BP3A.251005.004.B2)
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Description

Fixes #14750

Summary

This change adds input-level validation to the custom one-time donation amount field.

Previously, users could continue entering arbitrarily large numeric values into the custom donation amount field. This change prevents additional digits from being entered once the configured input limit has been reached while preserving the existing currency formatting and validation behavior.

Testing

Device

• Google Pixel 8
• Android 16 (Build: BP3A.251005.004.B2)

App

• Signal Android 8.17.1 (versionCode 171101)

Verified that:

• Valid custom donation amounts can still be entered.
• Currency formatting continues to work correctly.
• Additional digits are rejected once the configured input limit is reached.
• Existing predefined donation options continue to function as expected.

Screenshots

Before

After

[abgolor]

@jeffrey-signal @cody-signal I solved this issue but I am yet to get any feedback