Add packaging workflow and browser dist

Add a GitHub Actions workflow (package-extensions.yml) to build and upload Chrome/Firefox packages (store and offline artifacts) on manual trigger or when pushing v* tags. Update README with packaging instructions. Reorganize extension sources into chrome/ and firefox/ directories, add Firefox-specific files (manifest, locales, assets, CSS, LICENSE), and bump Chrome manifest version to 2026.04.23. Also modify js/oper.js (moved to chrome/js) to improve proportional editor resizing: add drag-to-resize, scale clamping/persistence (localStorage + chrome.storage.sync), pointer event handlers, and max-scale computation.

firefox/manifest.json

@@ -0,0 +1,43 @@
+{
+   "manifest_version": 2,
+   "name": "__MSG_extName__",
+   "default_locale": "en",
+   "version": "2026.4.23",
+   "browser_action": {
+      "default_popup": "popup.html",
+      "default_icon": "assets/logo_24x24.png",
+      "default_title": "__MSG_actionTitle__"
+   },
+   "description": "__MSG_extDescription__",
+   "homepage_url": "https://github.com/Jonnyan404/memos-bber",
+   "browser_specific_settings": {
+      "gecko": {
+         "id": "memos-bber@jonnyan404.github.io"
+      }
+   },
+   "icons": {
+      "128": "assets/logo.png",
+      "16": "assets/logo.png",
+      "48": "assets/logo.png"
+   },
+   "background": {
+      "scripts": ["js/background.js"]
+   },
+   "permissions": [
+      "tabs",
+      "storage",
+      "activeTab",
+      "contextMenus",
+      "http://*/*",
+      "https://*/*"
+   ],
+   "commands": {
+      "open-extension": {
+        "description": "Open my extension",
+        "suggested_key": {
+            "default": "Ctrl+Shift+F",
+            "mac": "MacCtrl+Shift+F"
+        }
+      }
+    }
+}