Use connectapi.garmin.com directly instead of web proxy

The connect.garmin.com/modern/proxy URL returns HTML (website) instead
of JSON API responses. Garth library uses connectapi.garmin.com subdomain
directly, which is the actual API endpoint.

- Change base URL from connect.garmin.com/modern/proxy to connectapi.garmin.com
- Update User-Agent to match garth library: GCM-iOS-5.19.1.2
- Factor out headers into getGarminHeaders() to avoid duplication
- Remove NK header (not needed when using connectapi subdomain)

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

src/lib/garmin.ts

@@ -4,10 +4,15 @@
 import { logger } from "@/lib/logger";
 import type { GarminTokens, HrvStatus } from "@/types";
 
-const GARMIN_BASE_URL = "https://connect.garmin.com/modern/proxy";
+// Use connectapi subdomain directly (same as garth library)
+const GARMIN_API_URL = "https://connectapi.garmin.com";
 
-interface GarminApiOptions {
-  oauth2Token: string;
+// Headers matching garth library's http.py USER_AGENT
+function getGarminHeaders(oauth2Token: string): Record<string, string> {
+  return {
+    Authorization: `Bearer ${oauth2Token}`,
+    "User-Agent": "GCM-iOS-5.19.1.2",
+  };
 }
 
 export interface BodyBatteryData {
@@ -17,14 +22,10 @@ export interface BodyBatteryData {
 
 export async function fetchGarminData(
   endpoint: string,
-  options: GarminApiOptions,
+  oauth2Token: string,
 ): Promise<unknown> {
-  const response = await fetch(`${GARMIN_BASE_URL}${endpoint}`, {
-    headers: {
-      Authorization: `Bearer ${options.oauth2Token}`,
-      NK: "NT",
-      "User-Agent": "GCM-iOS-5.7.2.1",
-    },
+  const response = await fetch(`${GARMIN_API_URL}${endpoint}`, {
+    headers: getGarminHeaders(oauth2Token),
   });
 
   if (!response.ok) {
@@ -58,12 +59,8 @@ export async function fetchHrvStatus(
   oauth2Token: string,
 ): Promise<HrvStatus> {
   try {
-    const response = await fetch(`${GARMIN_BASE_URL}/hrv-service/hrv/${date}`, {
-      headers: {
-        Authorization: `Bearer ${oauth2Token}`,
-        NK: "NT",
-        "User-Agent": "GCM-iOS-5.7.2.1",
-      },
+    const response = await fetch(`${GARMIN_API_URL}/hrv-service/hrv/${date}`, {
+      headers: getGarminHeaders(oauth2Token),
     });
 
     if (!response.ok) {
@@ -113,13 +110,9 @@ export async function fetchBodyBattery(
 ): Promise<BodyBatteryData> {
   try {
     const response = await fetch(
-      `${GARMIN_BASE_URL}/usersummary-service/stats/bodyBattery/dates/${date}`,
+      `${GARMIN_API_URL}/usersummary-service/stats/bodyBattery/dates/${date}`,
       {
-        headers: {
-          Authorization: `Bearer ${oauth2Token}`,
-          NK: "NT",
-          "User-Agent": "GCM-iOS-5.7.2.1",
-        },
+        headers: getGarminHeaders(oauth2Token),
       },
     );
 
@@ -172,13 +165,9 @@ export async function fetchIntensityMinutes(
 ): Promise<number> {
   try {
     const response = await fetch(
-      `${GARMIN_BASE_URL}/fitnessstats-service/activity`,
+      `${GARMIN_API_URL}/fitnessstats-service/activity`,
       {
-        headers: {
-          Authorization: `Bearer ${oauth2Token}`,
-          NK: "NT",
-          "User-Agent": "GCM-iOS-5.7.2.1",
-        },
+        headers: getGarminHeaders(oauth2Token),
       },
     );