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/src/pages/SettingsPage.tsx

@@ -2,10 +2,12 @@ import { AlertThresholdsCard } from "@/components/settings/AlertThresholdsCard";
 import { BackupCard } from "@/components/settings/BackupCard";
 import { CurrencyCard } from "@/components/settings/CurrencyCard";
 import { LanguageCard } from "@/components/settings/LanguageCard";
+import { ThemeCard } from "@/components/settings/ThemeCard";
 
 export function SettingsPage() {
   return (
     <div className="space-y-5">
+      <ThemeCard />
       <LanguageCard />
       <CurrencyCard />
       <AlertThresholdsCard />