Implement Garmin biometric fetching functions (P2.1)

Add specific fetchers for HRV, Body Battery, and Intensity Minutes
to enable real biometric data collection from Garmin Connect API.

Functions added:
- fetchHrvStatus(): Returns "Balanced", "Unbalanced", or "Unknown"
- fetchBodyBattery(): Returns current BB and yesterday's low value
- fetchIntensityMinutes(): Returns 7-day rolling sum of activity

All functions gracefully handle API failures with safe defaults.
Test count expanded from 14 to 33 covering all scenarios.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/lib/garmin.ts

@@ -1,6 +1,6 @@
 // ABOUTME: Garmin Connect API client using stored OAuth tokens.
 // ABOUTME: Fetches body battery, HRV, and intensity minutes from Garmin.
-import type { GarminTokens } from "@/types";
+import type { GarminTokens, HrvStatus } from "@/types";
 
 const GARMIN_BASE_URL = "https://connect.garmin.com/modern/proxy";
 
@@ -8,6 +8,11 @@ interface GarminApiOptions {
   oauth2Token: string;
 }
 
+export interface BodyBatteryData {
+  current: number | null;
+  yesterdayLow: number | null;
+}
+
 export async function fetchGarminData(
   endpoint: string,
   options: GarminApiOptions,
@@ -37,3 +42,101 @@ export function daysUntilExpiry(tokens: GarminTokens): number {
   const diffMs = expiresAt.getTime() - now.getTime();
   return Math.floor(diffMs / (1000 * 60 * 60 * 24));
 }
+
+export async function fetchHrvStatus(
+  date: string,
+  oauth2Token: string,
+): Promise<HrvStatus> {
+  try {
+    const response = await fetch(`${GARMIN_BASE_URL}/hrv-service/hrv/${date}`, {
+      headers: {
+        Authorization: `Bearer ${oauth2Token}`,
+        NK: "NT",
+      },
+    });
+
+    if (!response.ok) {
+      return "Unknown";
+    }
+
+    const data = await response.json();
+    const status = data?.hrvSummary?.status;
+
+    if (status === "BALANCED") {
+      return "Balanced";
+    }
+    if (status === "UNBALANCED") {
+      return "Unbalanced";
+    }
+    return "Unknown";
+  } catch {
+    return "Unknown";
+  }
+}
+
+export async function fetchBodyBattery(
+  date: string,
+  oauth2Token: string,
+): Promise<BodyBatteryData> {
+  try {
+    const response = await fetch(
+      `${GARMIN_BASE_URL}/usersummary-service/stats/bodyBattery/dates/${date}`,
+      {
+        headers: {
+          Authorization: `Bearer ${oauth2Token}`,
+          NK: "NT",
+        },
+      },
+    );
+
+    if (!response.ok) {
+      return { current: null, yesterdayLow: null };
+    }
+
+    const data = await response.json();
+
+    const currentData = data?.bodyBatteryValuesArray?.[0];
+    const current = currentData?.charged ?? null;
+
+    const yesterdayStats = data?.bodyBatteryStatList?.[0];
+    const yesterdayLow = yesterdayStats?.min ?? null;
+
+    return { current, yesterdayLow };
+  } catch {
+    return { current: null, yesterdayLow: null };
+  }
+}
+
+export async function fetchIntensityMinutes(
+  oauth2Token: string,
+): Promise<number> {
+  try {
+    const response = await fetch(
+      `${GARMIN_BASE_URL}/fitnessstats-service/activity`,
+      {
+        headers: {
+          Authorization: `Bearer ${oauth2Token}`,
+          NK: "NT",
+        },
+      },
+    );
+
+    if (!response.ok) {
+      return 0;
+    }
+
+    const data = await response.json();
+    const weeklyTotal = data?.weeklyTotal;
+
+    if (!weeklyTotal) {
+      return 0;
+    }
+
+    const moderate = weeklyTotal.moderateIntensityMinutes ?? 0;
+    const vigorous = weeklyTotal.vigorousIntensityMinutes ?? 0;
+
+    return moderate + vigorous;
+  } catch {
+    return 0;
+  }
+}