feat(app): panel zoom — full-grid render + header toggle

Wire Cmd::SetZoom through Tauri bridge (set_zoom command), add zoomed
field to WorkspaceView, short-circuit LayoutEngine to render only the
zoomed panel full-grid, and toggle Maximize2/Minimize2 in panel header.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src-tauri/src/lib.rs

@@ -49,6 +49,7 @@ pub fn run() {
             bridge::set_group,
             bridge::delete_group,
             bridge::focus,
+            bridge::set_zoom,
             bridge::event_log,
             bridge::mark_read,
             bridge::health,