Add User-Agent header to Garmin API requests

Garmin now requires a mobile app User-Agent header (GCM-iOS-5.7.2.1)
for API access. Without it, they serve the website HTML instead of
JSON API responses.

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

src/lib/garmin.test.ts

@@ -183,6 +183,7 @@ describe("fetchGarminData", () => {
         headers: {
           Authorization: "Bearer test-token",
           NK: "NT",
+          "User-Agent": "GCM-iOS-5.7.2.1",
         },
       },
     );

src/lib/garmin.ts

@@ -23,6 +23,7 @@ export async function fetchGarminData(
     headers: {
       Authorization: `Bearer ${options.oauth2Token}`,
       NK: "NT",
+      "User-Agent": "GCM-iOS-5.7.2.1",
     },
   });
 
@@ -61,6 +62,7 @@ export async function fetchHrvStatus(
       headers: {
         Authorization: `Bearer ${oauth2Token}`,
         NK: "NT",
+        "User-Agent": "GCM-iOS-5.7.2.1",
       },
     });
 
@@ -116,6 +118,7 @@ export async function fetchBodyBattery(
         headers: {
           Authorization: `Bearer ${oauth2Token}`,
           NK: "NT",
+          "User-Agent": "GCM-iOS-5.7.2.1",
         },
       },
     );
@@ -174,6 +177,7 @@ export async function fetchIntensityMinutes(
         headers: {
           Authorization: `Bearer ${oauth2Token}`,
           NK: "NT",
+          "User-Agent": "GCM-iOS-5.7.2.1",
         },
       },
     );