ui-kit(chart): ChartTooltipContent renders a zero value as a bare unstyled "0" instead of a formatted number

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⚠️ Definition of Done: this issue must be completed in full, in a single PR. Do not split this
work across multiple PRs, and do not defer any Deliverable below to a follow-up issue. A PR that
satisfies only some of the Deliverables, stubs a required test, or leaves a checkbox
partially-done does NOT resolve this issue and will be closed.

Context

ChartTooltipContent renders each series' value behind a truthiness check:

// packages/loopover-ui-kit/src/components/chart.tsx:252-256
                        {item.value && (
                          <span className="font-mono font-medium tabular-nums text-foreground">
                            {item.value.toLocaleString()}
                          </span>
                        )}

item.value is recharts' TooltipValueType — a number, a string, or an array. When it is the number 0,
0 && <span/> evaluates to 0, and React renders the number 0 as a raw text node in place of the
element. Two things go wrong at once:

1. The value loses font-mono font-medium tabular-nums text-foreground. In a tooltip whose whole point is
   tabular-nums alignment, one row renders in the surrounding body font while its siblings render monospaced.
2. It bypasses toLocaleString(), so it is not the formatted value the component promises — the same code path
   that turns 1200 into 1,200 is skipped.

An empty-string value ("") hits the same branch and renders nothing at all, which is at least silent; 0 is
the visible one. Zero is an ordinary data point for both live consumers —
apps/loopover-miner-ui/src/routes/ledgers.tsx and apps/loopover-miner-ui/src/routes/portfolio.tsx both
render ChartTooltipContent over count/earnings series where a zero bucket is normal.

packages/loopover-ui-kit/src/components/chart.test.tsx was added by #9937 precisely because chart.tsx
"shipped with no tests at all", but its payload fixtures use 120 and 80 only — the zero case is untested.

Requirements

• A numeric item.value of 0 must render inside the same <span className="font-mono font-medium tabular-nums text-foreground"> as any other number, with its text produced by toLocaleString() (so the
  rendered text is "0", inside that span).
• The value element must be omitted only when there is genuinely no value — item.value === undefined or
  item.value === null. Every other value, including 0 and "", must render through the span.
• The guard must be an explicit nullish check, not a truthiness check, so no falsy-but-real value can ever
  again fall through as a raw React child.
• The custom-formatter branch at packages/loopover-ui-kit/src/components/chart.tsx:212 must NOT change: it
  already guards on item?.value !== undefined && item.name, and a formatter supplied by the caller still
  takes precedence over the default rendering.
• No other part of the row (the indicator, the icon, the itemConfig?.label || item.name series label, the
  nestLabel layout) may change.

⚠️ Required pattern: replace the && guard at packages/loopover-ui-kit/src/components/chart.tsx:252 with
a nullish-aware conditional, mirroring the item?.value !== undefined check the sibling formatter branch
at line 212 already uses. What does NOT satisfy this issue: (a) {item.value != null && (...)} written as a
loose != without a test pinning both 0 and undefined, which is the same one-character-away mistake;
(b) coercing with String(item.value ?? "") and always rendering the span, which makes an
absent value render an empty monospaced box; (c) changing toLocaleString() to a hand-rolled formatter;
(d) a repo-wide sweep of every {x && <jsx/>} in the ui-kit — this issue is chart.tsx's tooltip value
only; (e) a test-only PR.

Deliverables

• Rendering <ChartTooltipContent active payload={[{ dataKey: "revenue", name: "revenue", value: 0, color: "#0ea5e9", payload: { month: "Jan", revenue: 0 } }]} /> inside a ChartContainer produces a
  <span> whose className contains tabular-nums and whose text is "0", asserted in
  packages/loopover-ui-kit/src/components/chart.test.tsx.
• Rendering the same payload entry with value: undefined produces no value <span> at all (the series
  label still renders), asserted in the same file.
• Rendering the same payload entry with value: "" produces the value <span> with empty text, asserted
  in the same file.
• The existing "renders one row per payload entry, labelled from the chart config" test (values 120 /
  80) still passes unmodified.
• A regression test at packages/loopover-ui-kit/src/components/chart.test.tsx named for this bug that
  asserts the zero value is NOT a direct text child of the row container — i.e. that the rendered 0 sits
  inside the formatted span, not beside it.

All Deliverables above are required in a single PR. A PR that satisfies only some of them — for example one
that changes the guard and asserts only that the text "0" appears somewhere (which the buggy code also
satisfies) — does not resolve this issue.

Test Coverage Requirements

This repo enforces 99%+ Codecov patch coverage, branch-counted. vitest.config.ts's coverage.include
covers src/**/*.ts, packages/loopover-engine/src/**/*.ts, packages/loopover-{miner,mcp}/{lib,bin}/**/*.ts,
packages/loopover-contract/src/**/*.ts and packages/discovery-index/src/**/*.ts. It does not cover
packages/loopover-ui-kit/**, and that package's own packages/loopover-ui-kit/vitest.config.ts deliberately
declares no coverage block. Codecov does not gate the patch on this change.

The tests are still mandatory and are named above: they go in the existing
packages/loopover-ui-kit/src/components/chart.test.tsx and must be run with
npm --workspace @loopover/ui-kit run test before pushing (that suite is not currently reached by
npm run test:ci, so a local run is the only way to see it pass).

Both arms of the changed guard need a test in that file: value present (0, "", and a non-zero number all
render the span) and value absent (undefined renders nothing). The sibling formatter branch's two arms must
also stay covered — the existing "routes the value through a custom formatter" test covers the formatter arm;
add or keep an assertion for the no-formatter arm with a zero value so the two branches cannot be conflated.

Expected Outcome

A chart data point of 0 shows as a properly formatted, monospaced, tabular-aligned 0 in the tooltip like
every other value, instead of an unstyled bare digit that breaks column alignment and skips
toLocaleString().

Links & Resources

• packages/loopover-ui-kit/src/components/chart.tsx:252-256 — the truthiness guard
• packages/loopover-ui-kit/src/components/chart.tsx:212 — the sibling formatter branch's nullish check
• packages/loopover-ui-kit/src/components/chart.test.tsx — the existing suite (fixtures 120/80 only)
• apps/loopover-miner-ui/src/routes/ledgers.tsx, apps/loopover-miner-ui/src/routes/portfolio.tsx — the
  two live consumers
• Ui-kit release due: 1.3.1 #9937, Evaluate recharts v3 migration for apps/loopover-ui #8610 — the test-backfill issue and the recharts v3 migration that rewrote this file

[linear-code]

JSO-7026