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/frontend/src/lib/theme/theme.svelte.ts

@@ -24,11 +24,11 @@ export const THEME_STORAGE_KEY = "galaxy-theme";
 const SYSTEM_LIGHT_QUERY = "(prefers-color-scheme: light)";
 
 function readStoredChoice(): ThemeChoice {
-	if (typeof localStorage === "undefined") return "dark";
+	if (typeof localStorage === "undefined") return "system";
 	const value = localStorage.getItem(THEME_STORAGE_KEY);
 	return value === "light" || value === "dark" || value === "system"
 		? value
-		: "dark";
+		: "system";
 }
 
 function systemTheme(): ResolvedTheme {