feat(metrics): tag cryptify uploads with X-Cryptify-Source: outlook (#96)

Adds the X-Cryptify-Source: outlook header to clientHeaders() so
cryptify's per-channel upload metrics
(encryption4all/cryptify#102) classify this add-in deterministically.

cryptify's detect_channel checks the Origin header before falling back
to User-Agent substring matching. The add-in is served from
addin.*.postguard.eu, which matches cryptify's `contains("postguard.")`
rule and would otherwise shadow the User-Agent "outlook" check —
labeling Outlook uploads as `website` / `staging-website` instead of
`outlook`. Setting the explicit header here removes the ambiguity.

Author: rubenhensen

src/lib/pkg-client.ts

@@ -15,5 +15,11 @@ export const CLIENT_ID = "pg4ol";
 export function clientHeaders(addinVersion: string): Record<string, string> {
   return {
     "X-PostGuard-Client-Version": `${CLIENT_NAME},1.0,${CLIENT_ID},${addinVersion}`,
+    // Identifies this add-in in cryptify's per-channel upload metrics.
+    // Required because cryptify's detect_channel checks the Origin
+    // header before User-Agent, and the add-in is served from
+    // addin.*.postguard.eu — without this header it would be
+    // misclassified as `website` / `staging-website`.
+    "X-Cryptify-Source": "outlook",
   };
 }