Fix packaged headless terminal dependencies

[parsakhaz]

Summary

• declare the headless xterm runtime packages in the root Electron app manifest
• regenerate the lockfile's root importer and NOTICES
• ensure Electron Builder includes the modules required by terminalPanelManager

Why

PR #336 added @xterm/headless and @xterm/addon-serialize to main/package.json, which is sufficient for workspace development and tests. The packaged application is rooted at the repository-level package.json, however, so Electron Builder omitted those runtime modules from the Linux AppImage. The v2.4.27 release's packaged headless smoke test then hung after Electron failed during module loading.

Testing

• pnpm install
• pnpm build (frontend, main, and universal macOS Electron package)
• inspected packaged app.asar and confirmed both /node_modules/@xterm/headless and /node_modules/@xterm/addon-serialize
• ran the packaged Pane.app/Contents/MacOS/Pane --remote-setup ... --no-install-service; it printed the connection configuration and exited successfully

Release Note

The v2.4.27 workflow was cancelled after its Linux packaged-headless smoke test exposed this missing dependency. Cut v2.4.28 after merge.