Fix Firefox install: use background.scripts instead of service_worker

Firefox MV3 does not support background.service_worker. Split background
into background-boot.js (globals) and background-main.js (startup logic);
Chrome keeps using importScripts() in background.js, Firefox manifest uses
background.scripts with the same file list so the extension loads in both.

extension/manifest_firefox.json

@@ -29,7 +29,16 @@
     }
   },
   "background": {
-    "service_worker": "background.js"
+    "scripts": [
+      "background/background-boot.js",
+      "background/logs.js",
+      "background/state.js",
+      "background/proxy.js",
+      "background/native-messaging.js",
+      "background/tab-lifecycle.js",
+      "background/message-router.js",
+      "background/background-main.js"
+    ]
   },
   "content_scripts": [
     {