Coverage: Matrix composables to 80% lines

[mjkatgithub]

Planning

• Milestone: Phase 4
• Priority: P2
• Size: L
• Estimate: 5 SP
• Epic: Epic: Raise unit coverage toward critical-path targets #129

Goal

Raise line coverage on app/composables/matrix/** toward ≥ 80% with focused
unit tests (not export-only checks).

Scope

• Prioritize untested branches in messages.ts, roomsOrDirectory.ts,
  sessionCrypto.ts, registration UIA helpers
• Extend existing useMatrixClient.*.spec.ts or add focused util specs
• Do not duplicate full integration scenarios

Branch

feature/coverage-matrix-composables

Acceptance Criteria

• app/composables/matrix/ ≥ 80% lines in coverage report
• npm run test:coverage green with current global thresholds
• No flaky parallel failures (keep pool: forks / mock isolation)

Test Checklist

Unit / coverage

• npm run test:unit
• npm run test:coverage

Notes

Depends on #129 baseline doc (optional but recommended). Docs: N/A unless
new test support files move module boundaries.