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/index.css

@@ -2,8 +2,9 @@
 
 /*
  * Semantic tokens, light values first. Dark values are re-declared for both
- * the OS-level `prefers-color-scheme` and an eventual manual `[data-theme]`
- * toggle, per the dataviz skill's reference palette (references/palette.md).
+ * the OS-level `prefers-color-scheme` and the manual `[data-theme]` toggle
+ * (Settings → Theme, see lib/theme.ts), per the dataviz skill's reference
+ * palette (references/palette.md).
  */
 :root {
   color-scheme: light;