feat(settings): add theme preference selector replacing header ModeToggle

tests/settings-panel.test.ts

@@ -0,0 +1,11 @@
+import { describe, expect, test } from "bun:test";
+import { readFileSync } from "node:fs";
+
+describe("settings panel", () => {
+	test("theme controls are available from settings after leaving the mobile header", () => {
+		const source = readFileSync("src/components/settings-panel.tsx", "utf8");
+
+		expect(source).toContain("Theme preference");
+		expect(source).toContain("setTheme(");
+	});
+});