fix(ui-kit): render chart tooltip zero inside tabular-nums span

[RealDiligent]

Summary

• Replace ChartTooltipContent's truthiness guard (item.value &&) with an explicit nullish check so numeric 0 renders inside the tabular-nums span via toLocaleString() instead of as a bare React text child.
• Empty string also goes through the span; only undefined / null omit the value element.
• Custom formatter branch unchanged.

Closes #10051

Test plan

• npm --workspace @loopover/ui-kit run test -- src/components/chart.test.tsx (13 pass)
• Zero value sits inside span.tabular-nums (not a direct text child)
• undefined / null omit the value span; "" renders empty span
• Existing 120/80 labelled-row test unmodified and still passing

[loopover-orb]

Tip

✅ LoopOver review result - approve/merge recommended

_{Review updated: 2026-07-31 07:54:23 UTC}

2 files · 1 AI reviewer · no blockers · readiness 86/100 · CI green · clean

✅ Suggested Action - Approve/Merge

• safe to merge

Review summary
This diff fixes a real bug: `item.value && (...)` in ChartTooltipContent skipped the tabular-nums span for falsy-but-valid values like `0` and `""`, rendering them as bare text or nothing. The replacement `item.value !== undefined && item.value !== null` is correct and matches the existing `formatter && item?.value !== undefined` pattern already used a few lines above in the same file. Tests added are well-targeted, exercise both the zero and empty-string cases and the undefined/null omission cases, and the diff correctly links to issue #10051.

Nits — 4 non-blocking

• chart.tsx:257 — for non-numeric values (e.g. strings), `.toLocaleString()` still works but has no real formatting effect; consider a comment noting `toLocaleString` is meant for numeric display only, though this isn't a regression introduced here.
• chart.test.tsx — the three new tests are somewhat verbose (particularly the DOM-traversal check for `directZero`); a simpler `container.textContent` vs `valueSpan.textContent` comparison would communicate the same invariant with less code.
• Consider consolidating the `undefined`/`null` check into a single `item.value == null` for brevity, though the explicit form is also fine and arguably clearer.
• The empty-string test asserts an empty span renders — worth double-checking this is desired product behavior (an invisible tabular-nums span) rather than also omitting it, since it's a UI-visible edge case not explicitly mentioned in the linked issue.

Decision drivers

• ✅ Code review — No blockers (1 reviewer)
• ✅ Gate result — Passing (No configured blocker found.)

Context & advisory signals — never blocks the verdict

Signal	Result	Evidence
Linked issue	✅ Linked	#10051
Related work	✅ No active overlap found	No same-issue or scoped active PR overlap found.
Change scope	❌ 8/20	High review scope from cached public metadata (1 linked issue).
Validation posture	✅ 25/25	PR body includes validation/test evidence.
Contributor workload	✅ 10/10	Author activity: 239 registered-repo PR(s), 89 merged, 38 issue(s).
Contributor context	✅ Confirmed Gittensor contributor	RealDiligent; Gittensor profile; 239 PR(s), 38 issue(s).
Improvement	✅ Minor	risk: clean · value: minor · LLM: minor

Linked issue satisfaction

Addressed
The diff replaces the truthy `&&` guard with an explicit `item.value !== undefined && item.value !== null` check, leaving the formatter branch untouched, and adds tests in chart.test.tsx covering zero rendering inside the tabular-nums span, undefined/null omission, and empty-string rendering as required by the deliverables.

Review context

• Author: RealDiligent
• Role context: outside_contributor
• Public audience mode: oss maintainer
• Lane context: Repository is configured for direct PR review.
• Public profile languages: Python, Ruby, TypeScript, Svelte, Cuda, JavaScript, Markdown, MDX
• Official Gittensor activity: 239 PR(s), 38 issue(s).
• PR-specific overlap: none found.

Contributor next steps

• Start here: Add a concise scope and risk note.
• Then work through the remaining 1 step in the Signals table above.

Signal definitions

• Related work = same linked issue, overlapping active PRs, or title/path similarity.
• Change scope = cached public metadata such as size labels, draft state, and review-burden hints.
• Validation posture = whether the PR provides enough public validation/test evidence for maintainer review.
• Contributor workload = public contributor activity and cleanup pressure, not a repo-wide quality failure.
• Contributor context = public GitHub/Gittensor identity context; non-Gittensor status is not a blocker.

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_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed}

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[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[loopover-orb]

LoopOver approves — the gate is satisfied and CI is green.