feat(metrics): tag cryptify uploads with X-Cryptify-Source: thunderbird (#121)

Adds X-Cryptify-Source: thunderbird to PG_CLIENT_HEADER so cryptify's
per-channel upload metrics (encryption4all/cryptify#102) classify this
add-in deterministically rather than via the User-Agent substring
fallback.

Thunderbird WebExtensions don't always present a stable Origin
(can be `moz-extension://<uuid>` or null), and the User-Agent fallback
is the last layer of cryptify's detect_channel. Setting the explicit
header at the source removes any environment-dependent ambiguity and is
symmetric with parallel PRs in postguard-website and
postguard-outlook-addon.

Author: rubenhensen

src/background/background.ts

@@ -171,6 +171,11 @@ const extVersion = browser.runtime.getManifest().version;
 
 PG_CLIENT_HEADER = {
   "X-PostGuard-Client-Version": `Thunderbird,${tbVersion},pg4tb,${extVersion}`,
+  // Identifies this add-in in cryptify's per-channel upload metrics
+  // (encryption4all/cryptify#102). Explicit header avoids relying on
+  // cryptify's Origin/User-Agent fallbacks, which are environment- and
+  // host-dependent.
+  "X-Cryptify-Source": "thunderbird",
 };
 
 console.log(`[PostGuard] v${extVersion} started (Thunderbird ${tbVersion})`);