Increase Jest branch coverage above 80% by adding edge-case tests

[cruddasj]

Motivation

• Raise Jest branch coverage to at least 80% for the client app so more code paths are exercised by unit tests.

Description

• Expanded tests/app.coverage-extra.test.js with multiple new scenarios covering update-flow fallbacks, modal behavior (overlay/escape/close button), malformed/failed changelog and version responses, navigation/brand-home paths, storage failures, and confirmed/cancelled clear-data flows.
• Updated the test DOM fixtures (added #updateButton and a close-modal button) to exercise UI flows without changing production code.
• Added changelog entries to CHANGELOG.md and assets/changelog.json using [NEXT_VERSION] / [NEXT_DATE] placeholders to document the testing/quality update.
• Left jest.config.js and coverage collection targeting assets/js/app.js unchanged per repository guidance.

Testing

• Ran npm outdated --json (no patch/minor dependency updates applied).
• Ran npm run lint which completed successfully (ESLint + markdownlint).
• Ran npm test -- --coverage and all tests passed (2 test suites, 27 tests in the final run) and coverage met the goal with branch coverage for assets/js/app.js above 80% (reported 81.22%).

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Codex Task

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	95.25% (+7.31% 🔼)	782/821
🟢	Branches	81.22% (+12.07% 🔼)	199/245
🟢	Functions	97.44% (+7.69% 🔼)	38/39
🟢	Lines	95.25% (+7.31% 🔼)	782/821

Test suite run success

27 tests passing in 2 suites.

Report generated by 🧪jest coverage report action from 3ef7824