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.

frontend/index.html

@@ -6,6 +6,24 @@
     <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover" />
     <meta name="theme-color" content="#0b0b0b" />
     <title>Aurum — Личные финансы</title>
+    <script>
+      // Applies the saved theme (light/dark/system) to <html> before the
+      // first paint, so an explicit override never flashes the wrong theme
+      // for a frame while React/lib/theme.ts loads. "system" needs no
+      // attribute at all — index.css's prefers-color-scheme media query
+      // already handles that case with zero JS.
+      (function () {
+        var stored = localStorage.getItem("aurum:theme");
+        if (stored === "light" || stored === "dark") {
+          document.documentElement.setAttribute("data-theme", stored);
+        }
+        var isDark =
+          stored === "dark" || (stored !== "light" && window.matchMedia("(prefers-color-scheme: dark)").matches);
+        var meta = document.querySelector('meta[name="theme-color"]');
+        // Kept in sync by hand with index.css's --surface-0 light/dark values.
+        if (meta) meta.setAttribute("content", isDark ? "#0d0d0d" : "#f9f9f7");
+      })();
+    </script>
   </head>
   <body>
     <div id="root"></div>