feat(reports): prior-period comparison toggle on the Sales report

Adds a "Compare to prior period" toggle to the Sales report header.
When on, the API recomputes the KPI window for the equal-length window
immediately preceding the selected range (previousPeriodBounds) behind
`?compare=1`, and the five window-derived KPI tiles (Won, Lost, Win
rate, Avg time-to-close, New leads) render colour-correct "vs prior"
deltas. Point-in-time tiles (Active interests, Pipeline value) have no
prior-window analogue and intentionally show no delta. The prior-window
query runs in parallel with the main batch and resolves to null when the
toggle is off (zero cost). Toggle state persists in the saved-template
config.

Closes the spec's "period comparison on every report" gap for Sales;
Operational already rendered period-start deltas.

Pure helpers TDD'd: previousPeriodBounds (range.ts) +
computeSalesKpiComparison (sales-comparison.ts), 7 unit tests. tsc +
lint clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/services/reports/sales-comparison.ts

@@ -0,0 +1,47 @@
+/**
+ * Period-comparison diffing for the Sales report KPI strip.
+ *
+ * DB-free + pure so it unit-tests without a database. The route computes
+ * the current-window KPIs and the prior-window KPIs (via
+ * `getSalesKpis(portId, previousPeriodBounds(range))`) and hands both
+ * here to produce the per-tile deltas the KpiCard renders as
+ * "vs prior period" arrows.
+ *
+ * Only the window-derived KPIs get a delta — the point-in-time tiles
+ * (Active interests, Pipeline value) have no prior-window analogue, so
+ * they are intentionally absent here.
+ */
+
+// Type-only import: erased at compile time, so this module does NOT pull
+// in the DB driver that `sales.service.ts` imports at runtime.
+import type { SalesKpis } from '@/lib/services/reports/sales.service';
+
+export interface SalesKpiDeltas {
+  /** current - prior */
+  wonInWindow: number;
+  /** current - prior */
+  lostInWindow: number;
+  /** current - prior */
+  newLeadsInWindow: number;
+  /** current - prior as a fraction [-1, 1]; null when either period has
+   *  no rate (denominator was 0). */
+  winRate: number | null;
+  /** current - prior in days; null when either period has no median
+   *  (sample too small). */
+  medianTimeToCloseDays: number | null;
+}
+
+function diffNullable(current: number | null, prior: number | null): number | null {
+  if (current === null || prior === null) return null;
+  return current - prior;
+}
+
+export function computeSalesKpiComparison(current: SalesKpis, prior: SalesKpis): SalesKpiDeltas {
+  return {
+    wonInWindow: current.wonInWindow - prior.wonInWindow,
+    lostInWindow: current.lostInWindow - prior.lostInWindow,
+    newLeadsInWindow: current.newLeadsInWindow - prior.newLeadsInWindow,
+    winRate: diffNullable(current.winRate, prior.winRate),
+    medianTimeToCloseDays: diffNullable(current.medianTimeToCloseDays, prior.medianTimeToCloseDays),
+  };
+}