docs: add screen-level test matrix for wallet flows

[habnark]

Closes #443

Contributors couldn't tell which screens require tests or what "done" looks like for visual verification, leading to under-tested mobile PRs (per the issue).

• Add docs/screen-test-matrix.md: every screen under app/ (verified against the actual file tree, not the older/incomplete docs/screen-inventory.md), grouped into the categories the issue calls out — onboarding (welcome/create/import/success), balance, send, receive/QR, activity, address book, vault, settings — plus diagnostics.
• Each row lists required test types (component/store/flow/visual), cross-references the actual existing test file(s) in tests/ and tests/ (verified by reading them, not guessing from filenames), and a coverage status. This surfaced real gaps: onboarding has zero automated coverage, Settings has zero, and a few screens (Receive, Vault Detail, Diagnostics) only have logic-level tests rather than screen-level ones.
• Visual verification expectations per section, cross-linking the existing UI State Catalogue, Accessibility Checklist, and Design System docs rather than duplicating them.
• Link the matrix from README.md and from CONTRIBUTING.md's testing section.

Summary

Closes #

Test Plan and Evidence

Self-Assessment

Complete the
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• Scope: The change matches the linked issue and contains no unrelated
  or unfinished work.
• Tests: Appropriate automated and manual evidence is included above,
  or a no-test justification is provided.
• CI: Relevant local checks pass and required CI checks pass on the
  latest commit.
• Documentation: Affected documentation, comments, examples,
  screenshots, and links are updated, or no update is needed.
• Known limitations: Limitations, assumptions, risks, skipped checks,
  and follow-up work are disclosed below.
• Acceptance criteria: Each issue criterion is mapped to implementation
  or verification evidence below.

CI Status

Documentation

Known Limitations

Acceptance Criteria

Acceptance criterion	Implementation or evidence	Status
		Pass / Not applicable

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