docs: add meaningful-change guide for mobile contributors (#514)

Closes #447

Small mobile PRs can change styling or a few lines without actually
addressing the requested behaviour in an issue, and merging doesn't
mean the work is payment-ready.

- Add docs/meaningful-change-guide.md: defines the bar for "meaningful"
  (requested behaviour, tests, screenshots, accessibility, acceptance
  criteria), with concrete incomplete-vs-acceptable PR examples drawn
  from this app's actual screens (Send validation, Receive copy
  button, Vault lock states) rather than generic examples, required
  testing evidence, and reviewer expectations.
- Cross-links the Evaluation Readiness Checklist, the (forthcoming)
  Self-Review Checklist, the Screen Test Matrix, and the Accessibility
  Checklist rather than duplicating their content.
- Link the guide from README.md.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: habnark