feat: add extension implementation and docs

Add manifest.json, sidepanel components, and scripts.
Include project assets and documentation files.
Remove placeholder blank file.

manifest.json

@@ -0,0 +1,46 @@
+{
+  "manifest_version": 3,
+  "name": "Auto Cover Generator",
+  "version": "1.0.0",
+  "description": "สร้างหน้าปกข่าว/Youtube อัตโนมัติด้วย Gemini AI",
+  "icons": {
+    "16": "icons/icon16.png",
+    "48": "icons/icon48.png",
+    "128": "icons/icon128.png"
+  },
+  "permissions": [
+    "activeTab",
+    "sidePanel",
+    "downloads",
+    "storage",
+    "scripting"
+  ],
+  "host_permissions": [
+    "*://gemini.google.com/*"
+  ],
+  "background": {
+    "service_worker": "scripts/background.js"
+  },
+  "content_scripts": [
+    {
+      "matches": [
+        "https://gemini.google.com/*"
+      ],
+      "js": [
+        "scripts/content.js"
+      ],
+      "run_at": "document_idle"
+    }
+  ],
+  "side_panel": {
+    "default_path": "sidepanel/warning.html"
+  },
+  "action": {
+    "default_icon": {
+      "16": "icons/icon16.png",
+      "48": "icons/icon48.png",
+      "128": "icons/icon128.png"
+    },
+    "default_title": "Auto Cover Generator"
+  }
+}