feat: manual theme picker (light/dark/system) in Settings

Theme previously followed only the OS's prefers-color-scheme, which
switches automatically on this user's Mac at sunset/sunrise. Adds an
explicit override in Settings: Light/Dark/System, System being the
previous (default) auto-follow-the-OS behavior.

Persisted client-side in localStorage (lib/theme.ts, same pattern as
language) rather than server-side like currency — it's a pure display
preference, and synchronous localStorage reads are what make the
anti-FOUC inline script in index.html possible. index.css already had the
[data-theme] CSS variables laid in ahead of time; only the JS to actually
set the attribute, and the Settings UI, were missing.

Covered by 2 new Playwright specs (theme-switcher.spec.ts) run against the
isolated e2e stack: explicit choice overrides a dark OS and survives
reload; switching back to "system" doesn't leave a stale forced theme.

e2e/tests/theme-switcher.spec.ts

@@ -0,0 +1,35 @@
+import { test, expect } from "@playwright/test";
+
+// The theme picker (Settings → Тема оформления): light/dark/system, with
+// system meant to follow the OS live and light/dark meant to override it.
+// See frontend/src/lib/theme.ts and index.html's anti-FOUC inline script.
+
+async function surface0(page: import("@playwright/test").Page): Promise<string> {
+  return page.evaluate(() => getComputedStyle(document.documentElement).getPropertyValue("--surface-0").trim());
+}
+
+test.describe("theme picker", () => {
+  test.use({ colorScheme: "dark" }); // simulates the OS being in dark mode
+
+  test("an explicit choice overrides the OS theme, and persists across reload", async ({ page }) => {
+    await page.goto("/settings");
+    await expect(await surface0(page)).toBe("#0d0d0d"); // "system" follows the dark OS by default
+
+    await page.getByRole("button", { name: "Светлая", exact: true }).click();
+    await expect(await surface0(page)).toBe("#f9f9f7"); // forced light despite the dark OS
+
+    await page.reload();
+    await expect(await surface0(page)).toBe("#f9f9f7"); // survives a reload, not just in-memory state
+
+    await page.getByRole("button", { name: "Системная", exact: true }).click();
+    await expect(await surface0(page)).toBe("#0d0d0d"); // back to following the (still-dark) OS
+  });
+
+  test("switching back to system never leaves a stale forced theme behind", async ({ page }) => {
+    await page.goto("/settings");
+    await page.getByRole("button", { name: "Тёмная", exact: true }).click();
+    await page.getByRole("button", { name: "Системная", exact: true }).click();
+    const dataTheme = await page.evaluate(() => document.documentElement.getAttribute("data-theme"));
+    expect(dataTheme).toBeNull();
+  });
+});