fix: validate restore birthday height as integer with lower bound

[umeeSthein]

Summary

This change hardens birthday height validation in the restore flow.

Problem

The restore birthday height screen accepted user input as BigDecimal and converted it with toLong(). That left a few bad cases:

• decimal values could be interpreted incorrectly instead of being rejected
• very large values could fail conversion in an unsafe way
• invalid input did not surface a clear user-facing error
• values below saplingActivationHeight were not explained well to the user

Solution

This PR:

• adds a dedicated birthday height validator
• accepts only whole-number input
• uses exact long conversion so fractional and overflow values are rejected
• enforces the lower bound at saplingActivationHeight
• shows explicit error text in the restore birthday height field
• adds tests for empty input, separator-only input, decimal input, lower bound, boundary values, and overflow

Notes

• upper-bound validation against the current network height is intentionally left out to keep this PR small and focused
• tests were added, but local execution may depend on Java/Android environment availability