feat(ui): default theme to system (follow OS light/dark)

Light has been signed off, so the theme store's default choice is now
`system` (it was `dark` during the incremental migration). This matches
the app.html pre-paint guard, which already resolved an unset choice via
prefers-color-scheme — removing the brief boot-time mismatch where the
store re-pinned dark. Users still pin light/dark via the account-menu
picker. Updates the store default + its test and the design-system /
finalize-plan docs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/PLAN-finalize.md

@@ -25,9 +25,9 @@ Merged to `development` via PR #26 (2026-05-22): a shared design-token
 system (`ui/frontend/src/lib/theme/`), light/dark theming with a picker
 and pre-paint guard, and the whole UI migrated onto the tokens.
 Documented literal exceptions: the battle-scene data-viz palette, overlay
-scrims, and directional/deliberate drop shadows. The default theme is
-dark; flipping it to `system` is an available follow-up. Tokens and
-conventions live in `ui/docs/design-system.md`.
+scrims, and directional/deliberate drop shadows. The default theme
+follows the OS (`system`); the account-menu picker pins light or dark.
+Tokens and conventions live in `ui/docs/design-system.md`.
 
 Goal: replace the ad-hoc per-component styling (inline hex colors like
 `#0a0e1a`, one-off spacing) with a shared design language so every view