test: add dark-mode contrast, mempool state, referral link, and contract error coverage

[ACOB-DEV]

Description

Adds test coverage and supporting logic modules for four open issues: dark-mode contrast regressions on charts and badges, mempool visualizer latency and stalled-state classification, referral dashboard canonical link attribution, and transaction failure modal mapping for new contract error codes.

Closes #826
Closes #827
Closes #831
Closes #838

Changes proposed

What were you told to do?

• Add dark-mode contrast checks for chart legends and status badges #838: Validate contrast ratios for chart legend and badge colors in dark mode; add regression tests
• Add mempool visualizer latency and stalled-transaction state coverage #831: Add latency buckets and stalled-transaction state logic; cover delayed inclusion and resubmission states
• Add referral dashboard canonical link and attribution tests #827: Define canonical referral link normalization; preserve attribution parameters through copy/share; add tests
• Add transaction failure modal coverage for newly introduced contract error codes #826: Expand transaction failure modal error code mapping for newly introduced contract errors; add unknown-code graceful degradation tests

What did I do?

Dark-mode contrast checks (#838)

• Added client/src/components/charts/darkModeContrast.ts with WCAG contrast ratio helpers and theme color tokens for chart legends and status badges (active / warning / error)
• Added darkModeContrast.test.ts with 17 tests covering ratio accuracy, AA large/normal thresholds, and regression guards for each badge tone

Mempool visualizer latency and stalled-state (#831)

• Added client/src/components/mempool_graph/mempoolState.ts with pure logic: classifyTxState, getLatencyBucket, buildTxWithState, isStalledWithResubmission, partitionByState
• Added mempoolState.test.ts with 20 tests covering healthy/delayed/stalled classification, latency buckets, threshold edge cases, resubmission tracking, and partition grouping

Referral canonical link and attribution (#827)

• Added client/src/features/referrals/referralLink.ts with buildReferralLink, parseReferralParam, normalizeReferralLink, isAttributionPreserved
• Added referralLink.test.ts with 18 tests covering canonical URL construction, encoding, extra-param stripping, hash stripping, and attribution preservation across copy/share

Transaction failure modal error codes (#826)

• Added client/src/services/contractErrors.ts mapping 10 contract error codes (including new: STALE_PRICE_FEED, EPOCH_NOT_SETTLED, DUPLICATE_DEPOSIT) to UI-friendly title, description, and actionability; graceful degradation for unknown/future codes
• Added contractErrors.test.ts with 21 tests covering every known code, unknown/future code degradation, and schema integrity checks

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• This PR does not contain plagiarized content.
• The title and description of the PR is clear and explains the approach.
• I am making a pull request against the main branch (left side).
• My commit messages styles matches our requested structure.
• The implementation was reviewed and confirmed to work as intended.
• I am only making changes to files I was requested to.

Screenshots / Validation Evidence

All 76 new tests pass alongside the 19 pre-existing zap tests — 95 tests total, 0 failures.

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[edehvictor]

Nice implementation!

[edehvictor]

Kindly resolve conflict

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