Enhanced VS Code Marketplace - Antigravity Installer

Adds an "Install (Antigravity)" button to VS Code Marketplace extension pages, using the antigravity-ide:// protocol.

Enhanced VS Code Marketplace - Antigravity Installer is for developers who use both VS Code Marketplace and Antigravity IDE. VS Code Marketplace item pages only ship one "Install" button, which uses the `vscode:` protocol to send the extension to VS Code. This extension injects a second button next to it, with the same look and dimensions, but rewrites the link to use the `antigravity-ide:` protocol, so the same extension can be installed into Antigravity IDE with one click. Key features: - Adds an "Install (Antigravity)" button on VS Code Marketplace item pages (`https://marketplace.visualstudio.com/items?itemName=*`). - Clones the original "Install" button's size, padding, font, and layout so the marketplace UI stays intact. - Antigravity-styled appearance: black background, white border, and a rainbow gradient label. - Re-injects the button when VS Code Marketplace navigates between items via its SPA router. - Runs only on VS Code Marketplace item pages; other websites are not affected. Privacy notes: - The content script only runs on VS Code Marketplace item pages. - The extension does not read or store any cookies. - The extension does not make any network requests. - The extension does not collect, store, or upload user data. - The extension does not include analytics SDKs, ad SDKs, or remote code.