GTK: collapse sidebar widget sync into a panel-DA lookup table

[JDonaghy]

Problem

sync_sidebar_widgets() has a 30+ line match p { SidebarPanel::Git => { da.grab_focus() } ... } block that maps each panel variant to its DrawingArea ref for focus grab, plus a parallel loop for panel box visibility. Every new panel requires adding another arm.

Proposed fix

Store panel DAs in a HashMap<&str, Rc<RefCell<Option<DrawingArea>>>> keyed by panel ID string (e.g. "panel:git" → git_sidebar_da_ref). The focus-grab and visibility loops become a single lookup:

if let Some(da_ref) = self.panel_da_map.get(panel_id) {
    if let Some(ref da) = *da_ref.borrow() {
        da.grab_focus();
    }
}

Same for panel box visibility — a parallel panel_box_map.

Context

Identified during #385 GTK sidebar migration (Session 375). The match arms are boilerplate that scales linearly with panel count.