Add budgets view with progress bars and management

Month selector, summary of total budgeted vs spent, per-category
progress bars with green/yellow/red color coding based on percentage.
Add budget dialog with category picker and amount. Edit and delete
budgets with confirmation.

outlay-gtk/src/window.rs

@@ -2,6 +2,7 @@ use adw::prelude::*;
 use outlay_core::db::Database;
 use std::rc::Rc;
 
+use crate::budgets_view::BudgetsView;
 use crate::charts_view::ChartsView;
 use crate::history_view::HistoryView;
 use crate::log_view::LogView;
@@ -53,8 +54,12 @@ impl MainWindow {
         let charts_view = ChartsView::new(db.clone());
         content_stack.add_named(&charts_view.container, Some("charts"));
 
+        // Budgets view
+        let budgets_view = BudgetsView::new(db.clone());
+        content_stack.add_named(&budgets_view.container, Some("budgets"));
+
         // Remaining pages are placeholders for now
-        for item in &SIDEBAR_ITEMS[3..] {
+        for item in &SIDEBAR_ITEMS[4..] {
             let page = adw::StatusPage::builder()
                 .title(item.label)
                 .icon_name(item.icon)