feat(cycle): field-explainer tooltip + log-sheet i18n scaffold

Add a cycle-scoped FieldInfo affordance (a focusable, SR-reachable info
button with a 44 px touch target, mirroring the mood-explainer precedent)
and the i18n keys the redesigned log-day sheet needs: a cycle.fieldInfo.*
namespace of descriptive-only explainers (BBT, cervical mucus, ovulation
test, spotting, pregnancy/progesterone test, intercourse, protection) plus
cycle.sheet.phaseDayHeader and cycle.sheet.bbtHint, across all six locales.

src/components/cycle/field-info.tsx

@@ -0,0 +1,61 @@
+"use client";
+
+import { Info } from "lucide-react";
+
+import {
+  Tooltip,
+  TooltipContent,
+  TooltipProvider,
+  TooltipTrigger,
+} from "@/components/ui/tooltip";
+import { cn } from "@/lib/utils";
+
+/**
+ * v1.18.1 — the cycle log-sheet field-explainer affordance.
+ *
+ * A small "?" / info icon that puts a factual, descriptive-only one-liner
+ * (what a clinical field means — BBT, cervical mucus, an LH surge) right at
+ * the point of capture, so a first-time user never has to leave the sheet to
+ * understand a term. Mirrors the mood surface's `MoodExplainerIcon` precedent
+ * (self-contained `TooltipProvider`, a real focusable `<button>` trigger with
+ * an `aria-label`, keyboard- and SR-reachable via Radix) but bumps the hit
+ * area to ≥44 px for touch, since the cycle sheet is a thumb-first surface.
+ *
+ * Copy stays descriptive-only — never diagnosis or medical advice — matching
+ * the phase-education honesty gate.
+ */
+export function FieldInfo({
+  label,
+  detail,
+  className,
+}: {
+  /** Accessible name for the trigger (what the icon explains). */
+  label: string;
+  /** The explanation shown in the tooltip. */
+  detail: string;
+  className?: string;
+}) {
+  return (
+    <TooltipProvider delayDuration={150}>
+      <Tooltip>
+        <TooltipTrigger
+          type="button"
+          aria-label={label}
+          data-slot="cycle-field-info"
+          className={cn(
+            // size-6 icon inside an 11-unit (44 px) hit box so touch targets
+            // clear the WCAG 2.5.8 / Apple HIG minimum without enlarging the
+            // visible glyph.
+            "text-muted-foreground hover:text-foreground focus-visible:ring-ring -m-2.5 inline-flex size-11 shrink-0 items-center justify-center rounded-full p-2.5 transition-colors focus-visible:ring-2 focus-visible:outline-none",
+            className,
+          )}
+        >
+          <Info className="size-4" aria-hidden="true" />
+        </TooltipTrigger>
+        <TooltipContent className="max-w-[16rem] text-xs leading-snug">
+          {detail}
+        </TooltipContent>
+      </Tooltip>
+    </TooltipProvider>
+  );
+}