Add borders to windows; add navigation keybinds

Right now, colors are hardcoded in the Config in main.zig.

This commit also adds a couple of new keybinds for navigating between
windows. All keybinds are hardcoded as well right now.

src/Config.zig

@@ -0,0 +1,13 @@
+// SPDX-FileCopyrightText: 2026 Ben Buhse <me@benbuhse.email>
+//
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+const Config = @This();
+
+border_width: u3,
+border_color_focused: RiverColor,
+border_color_unfocused: RiverColor,
+
+const std = @import("std");
+
+const RiverColor = @import("utils.zig").RiverColor;