feat(web): design system — ms-* tokens, ThemeProvider, MosaicLogo, sidebar (#221)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/web/src/providers/theme-provider.tsx

@@ -0,0 +1,62 @@
+'use client';
+
+import { createContext, useContext, useEffect, useMemo, useState, type ReactNode } from 'react';
+
+export type Theme = 'dark' | 'light';
+
+interface ThemeContextValue {
+  theme: Theme;
+  toggleTheme: () => void;
+  setTheme: (theme: Theme) => void;
+}
+
+const STORAGE_KEY = 'mosaic-theme';
+const ThemeContext = createContext<ThemeContextValue | undefined>(undefined);
+
+function getInitialTheme(): Theme {
+  if (typeof document === 'undefined') {
+    return 'dark';
+  }
+
+  return document.documentElement.getAttribute('data-theme') === 'light' ? 'light' : 'dark';
+}
+
+export function ThemeProvider({ children }: { children: ReactNode }): React.JSX.Element {
+  const [theme, setThemeState] = useState<Theme>(getInitialTheme);
+
+  useEffect(() => {
+    document.documentElement.setAttribute('data-theme', theme);
+    window.localStorage.setItem(STORAGE_KEY, theme);
+  }, [theme]);
+
+  useEffect(() => {
+    const storedTheme = window.localStorage.getItem(STORAGE_KEY);
+    if (storedTheme === 'light' || storedTheme === 'dark') {
+      setThemeState(storedTheme);
+      return;
+    }
+
+    document.documentElement.setAttribute('data-theme', 'dark');
+  }, []);
+
+  const value = useMemo<ThemeContextValue>(
+    () => ({
+      theme,
+      toggleTheme: () => setThemeState((current) => (current === 'dark' ? 'light' : 'dark')),
+      setTheme: (nextTheme) => setThemeState(nextTheme),
+    }),
+    [theme],
+  );
+
+  return <ThemeContext.Provider value={value}>{children}</ThemeContext.Provider>;
+}
+
+export function useTheme(): ThemeContextValue {
+  const context = useContext(ThemeContext);
+
+  if (!context) {
+    throw new Error('useTheme must be used within ThemeProvider');
+  }
+
+  return context;
+}