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/docs/design-system.md

@@ -72,7 +72,8 @@ the colour block in `tokens.css`.
   persists the choice, applies `data-theme`, and — while the choice is
   `system` — follows OS theme changes via `matchMedia`.
 - The account menu (`account-menu.svelte`) exposes the picker. The
-  default is `dark`; `system` follows the OS.
+  default is `system` (it follows the OS preference); `light` / `dark`
+  pin a theme.
 
 The `app.html` guard and the store deliberately duplicate the
 resolution logic (one runs before modules load, the other after) — keep
@@ -115,6 +116,5 @@ The only remaining literal colours are the documented exceptions above:
 the battle-scene data-viz palette, the overlay scrims, and the
 directional / deliberate drop shadows.
 
-The default theme is **dark** while light coherence is being verified
-across the migrated views; once the owner signs off on light, the
-default can flip to `system`.
+The default theme is **`system`** — it follows the OS light/dark
+preference; users can pin light or dark via the account-menu picker.