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/utils.zig

@@ -0,0 +1,67 @@
+// SPDX-FileCopyrightText: 2026 Ben Buhse <me@benbuhse.email>
+//
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+pub const RiverColor = struct {
+    red: u32,
+    green: u32,
+    blue: u32,
+    alpha: u32,
+};
+
+/// Parse a color in the format 0xRRGGBB or 0xRRGGBBAA and convert it to
+/// 32-bit color values (used by Window.set_borders in rwm).
+pub fn parseRgba(s: []const u8) !RiverColor {
+    if (s.len != 8 and s.len != 10) return error.InvalidRgba;
+    if (s[0] != '0' or s[1] != 'x') return error.InvalidRgba;
+
+    // If the color is 0xRRGGBB, add FF for the alpha channel
+    var color = try fmt.parseUnsigned(u32, s[2..], 16);
+    if (s.len == 8) {
+        color <<= 8;
+        color |= 0xff;
+    }
+
+    const bytes: [4]u8 = @as([4]u8, @bitCast(color));
+    return parseRgbaHelper(bytes);
+}
+
+/// Parse a color in the format 0xRRGGBB or 0xRRGGBBAA and convert it to
+/// 32-bit color values (used by Window.set_borders in rwm) at comptime.
+pub fn parseRgbaComptime(comptime s: []const u8) RiverColor {
+    if (s.len != 8 and s.len != 10) @compileError("Invalid RGBA");
+    if (s[0] != '0' or s[1] != 'x') @compileError("Invalid RGBA");
+
+    // If the color is 0xRRGGBB, add FF for the alpha channel
+    comptime var color = try fmt.parseUnsigned(u32, s[2..], 16);
+    if (s.len == 8) {
+        color <<= 8;
+        color |= 0xff;
+    }
+
+    const bytes = @as([4]u8, @bitCast(color));
+    return parseRgbaHelper(bytes);
+}
+
+fn parseRgbaHelper(bytes: [4]u8) RiverColor {
+    const r: u32 = bytes[3];
+    const g: u32 = bytes[2];
+    const b: u32 = bytes[1];
+    const a: u32 = bytes[0];
+
+    // To convert from an 8-bit color to 32-bit color, we need to do
+    //     color * 2^32 / 2^8
+    // which is equivalent to
+    //     color * 2^24
+    // or, in other words,
+    //     color << 24
+    return .{
+        .red = r << 24,
+        .green = g << 24,
+        .blue = b << 24,
+        .alpha = a << 24,
+    };
+}
+
+const std = @import("std");
+const fmt = std.fmt;