feat: add Antigravity as an "Open in editor" target (#1424)

Registers the Google Antigravity IDE so the workspace Open-in-editor
picker detects and launches it (PATH probe for the "antigravity"
command), alongside Cursor, VS Code, WebStorm, and Zed.

- desktop: add the target to BUILT_IN_EDITOR_TARGETS (kind: editor).
- app: add "antigravity" to the known editor-target id set and map its
  bundled icon.

Co-authored-by: Mathias Kurz <mkurz@stamus-networks.com>

packages/app/src/workspace/editor-targets.ts

@@ -5,6 +5,7 @@ const KNOWN_EDITOR_TARGET_IDS: ReadonlySet<string> = new Set([
   "vscode",
   "webstorm",
   "zed",
+  "antigravity",
   "finder",
   "explorer",
   "file-manager",