fix(webapp): treat the White theme as light, not dark

useThemeMode only counted `light` itself as a light theme, so White fell
through to the dark branch. The toast rendered dark on a white page, and the
agent logo picked its dark dot palette.

It now classifies by theme family, reusing the enum that drives the "Light"
end of the `system` preference so a new theme is only classified once.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: samejr

apps/webapp/app/hooks/useThemeMode.ts

@@ -1,7 +1,13 @@
 import { useEffect, useState } from "react";
+import { SystemLightTheme } from "~/utils/themePreference";
 
 export type ThemeMode = "dark" | "light";
 
+/* Which themes read as light. Taken from the enum that also drives the "Light"
+   end of the `system` preference, so a new theme only has to be classified once
+   - anything not in here (dark, classic, black) reads as dark. */
+const LIGHT_THEMES = new Set<string>(SystemLightTheme.options);
+
 /**
  * The active theme's mode, for colors that can't come from a CSS variable. Resolved in an
  * effect so server and hydration renders agree; `root.tsx` can flip `data-theme` pre-paint.
@@ -10,7 +16,8 @@ export function useThemeMode(): ThemeMode {
   const [mode, setMode] = useState<ThemeMode>("dark");
   useEffect(() => {
     const resolve = () => {
-      setMode(document.documentElement.getAttribute("data-theme") === "light" ? "light" : "dark");
+      const theme = document.documentElement.getAttribute("data-theme");
+      setMode(theme !== null && LIGHT_THEMES.has(theme) ? "light" : "dark");
     };
     resolve();
     const observer = new MutationObserver(resolve);