chore: implement cross-browser build pipeline for extension

- Extract manifest.json to manifest.base.json
- Add Node.js build script to compile Chrome and Firefox artifacts
- Remove legacy sync-constants bat/sh scripts
- Update GitHub Actions workflow to use new build pipeline

extension/manifest.base.json

@@ -0,0 +1,37 @@
+{
+  "manifest_version": 3,
+  "name": "KoalaSync",
+  "version": "1.2.1",
+  "description": "Synchronize video playback across different tabs and users.",
+  "permissions": [
+    "storage",
+    "tabs",
+    "scripting",
+    "alarms",
+    "activeTab",
+    "notifications"
+  ],
+  "host_permissions": [
+    "<all_urls>"
+  ],
+  "action": {
+    "default_popup": "popup.html",
+    "default_icon": {
+      "16": "icons/icon128.png",
+      "48": "icons/icon128.png",
+      "128": "icons/icon128.png"
+    }
+  },
+  "content_scripts": [
+    {
+      "matches": ["https://koalasync.shik3i.net/*"],
+      "js": ["bridge.js"],
+      "run_at": "document_start"
+    }
+  ],
+  "icons": {
+    "16": "icons/icon128.png",
+    "48": "icons/icon128.png",
+    "128": "icons/icon128.png"
+  }
+}