fix(sf-legacy): treat velocity=0 as 'no data' in MarketTab

User reports on /legacy/site-finder/[cad]:
  Велосити 0.0 м²/мес · Срок продаж —
which is contradictory — if velocity were truly zero, sales period
would be infinity. Root cause: backend returns 0 (not null) when
competitors are unmapped to Objective ground truth (OBJ-3). The
condition for 'unavailable' must catch both null and zero.

Fix: getSalesPeriodMonths already treats velocitySqm <= 0 as null
(line 41), but the HeadlineBar + KPI card read raw velocity and
display '0.0 м²/мес' for zero. Same coercion applied at the source:
velocityPerMonth = (raw != null && raw > 0) ? raw : null.

Both HeadlineBar and KPI card now show '—' for zero velocity,
matching the '—' shown for Срок продаж.

frontend/src/components/site-finder/MarketTab.tsx

@@ -60,7 +60,12 @@ export function MarketTab({ data }: Props) {
   // competitors count as proxy for active (строящихся) count for now.
   const activeCount = data.competitors.length;
   const radiusKm = getRadiusKm(data);
-  const velocityPerMonth = data.velocity?.monthly_velocity_sqm ?? null;
+  // Treat zero velocity as "no data" — backend returns 0 when competitors
+  // are unmapped to Objective ground truth (OBJ-3). Showing "0.0 м²/мес"
+  // is misleading; "—" makes it clear data is unavailable.
+  const velocityRaw = data.velocity?.monthly_velocity_sqm;
+  const velocityPerMonth =
+    velocityRaw != null && velocityRaw > 0 ? velocityRaw : null;
   const maxMixPct = getMaxMixPct(data);
   const salesPeriodMonths = getSalesPeriodMonths(data);