[Cashtab] Match app send min to backend send min with constant

Summary: Matches front-end validation for min send amount with backend dust limit set by a constant in Ticker.js. Builds on validation tests and prep work from D9045 and D9021.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9052