Add Firefox-specific manifest and split packaging for Chrome zip vs XPI

- Add extension/manifest_firefox.json (no key, moz-extension://*/* for
  web_accessible_resources) for correct Firefox MV3 behavior
- Update pack-extension.js: zip excludes manifest_firefox.json; XPI
  built separately with manifest_firefox.json as manifest.json
- Update generate-extension-id.js to set gecko.id in manifest_firefox.json
  when present (no key in Firefox manifest)
- Document dual-manifest layout and pack/generate-id behavior in CONTRIBUTING.md

extension/manifest_firefox.json

@@ -0,0 +1,92 @@
+{
+  "manifest_version": 3,
+  "name": "Holesail Browser",
+  "version": "1.0.0",
+  "description": "P2P Holesail tunnels in the browser via native messaging host — open hs:// URLs with valid certificates",
+  "permissions": [
+    "nativeMessaging",
+    "notifications",
+    "tabs",
+    "declarativeNetRequest",
+    "proxy"
+  ],
+  "commands": {
+    "_execute_action": {
+      "suggested_key": {
+        "default": "Alt+Shift+H",
+        "mac": "Alt+Shift+H"
+      },
+      "description": "Open Holesail Dashboard"
+    }
+  },
+  "action": {
+    "default_title": "Holesail Dashboard",
+    "default_icon": {
+      "16": "icons/16.png",
+      "32": "icons/32.png",
+      "48": "icons/48.png",
+      "128": "icons/128.png"
+    }
+  },
+  "background": {
+    "service_worker": "background.js"
+  },
+  "content_scripts": [
+    {
+      "matches": [
+        "http://*/*",
+        "https://*/*"
+      ],
+      "js": [
+        "content.js"
+      ],
+      "run_at": "document_start"
+    }
+  ],
+  "web_accessible_resources": [
+    {
+      "resources": [
+        "vendor/xterm.js",
+        "vendor/xterm-addon-fit.js",
+        "vendor/xterm.css",
+        "vendor/novnc.js",
+        "wrong-domain.html",
+        "dashboard/dashboard.html",
+        "dashboard/manifest.webmanifest",
+        "dashboard/sw.js",
+        "dashboard/register-sw.js",
+        "dashboard/screenshots/*.png",
+        "dashboard/dashboard.css",
+        "dashboard/data/*.js",
+        "dashboard/core/*.js",
+        "dashboard/ui/*.js",
+        "dashboard/pages/*.js",
+        "dashboard/refresh.js",
+        "dashboard/events.js",
+        "background/*.js"
+      ],
+      "matches": [
+        "moz-extension://*/*"
+      ]
+    }
+  ],
+  "icons": {
+    "16": "icons/16.png",
+    "32": "icons/32.png",
+    "48": "icons/48.png",
+    "128": "icons/128.png"
+  },
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "[email protected]",
+      "strict_min_version": "109.0"
+    }
+  },
+  "host_permissions": [
+    "*://*.host.local/*",
+    "*://*.hole.sail/*"
+  ],
+  "optional_host_permissions": [
+    "*://*/*"
+  ]
+}