Add "Edit in VS Code" button to the code preview

Code/text files get a VS Code action in the preview header; a backend
open_in_vscode command resolves the depot file to its local working copy
and launches `code <path>` via the shell.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/p4.ts

@@ -74,6 +74,7 @@ export const p4 = {
   openInExplorer: (target: string) => invoke<void>("open_in_explorer", { target }),
   findUproject: (scope: string) => invoke<string>("find_uproject", { scope }),
   launchFile: (path: string) => invoke<void>("launch_file", { path }),
+  openInVscode: (depot: string) => invoke<void>("open_in_vscode", { depot }),
   readDepot: async (depot: string): Promise<ArrayBuffer> => {
     const r: unknown = await invoke("read_depot", { depot });
     if (r instanceof ArrayBuffer) return r;