Add charts view with pie, bar, and line charts

Pie chart shows expense breakdown by category with color-coded slices.
Bar chart shows income vs expense for the last 6 months. Line chart
shows cumulative daily net trend. All charts render via plotters
BitMapBackend to GDK MemoryTexture for display. Month navigation
shared across all three charts.

outlay-gtk/src/window.rs

@@ -2,6 +2,7 @@ use adw::prelude::*;
 use outlay_core::db::Database;
 use std::rc::Rc;
 
+use crate::charts_view::ChartsView;
 use crate::history_view::HistoryView;
 use crate::log_view::LogView;
 
@@ -48,8 +49,12 @@ impl MainWindow {
             .build();
         content_stack.add_named(&history_scroll, Some("history"));
 
+        // Charts view
+        let charts_view = ChartsView::new(db.clone());
+        content_stack.add_named(&charts_view.container, Some("charts"));
+
         // Remaining pages are placeholders for now
-        for item in &SIDEBAR_ITEMS[2..] {
+        for item in &SIDEBAR_ITEMS[3..] {
             let page = adw::StatusPage::builder()
                 .title(item.label)
                 .icon_name(item.icon)