docs: update documentation for v0.4.0 release

- Update README.md with v0.4.0 features and screenshots placeholders
- Update architecture.md with v0.4.0 implementation status
- Update progress.md marking all 27 tasks as completed
- Create CHANGELOG.md with complete release notes
- Add v0.4.0 frontend components and hooks

frontend/src/providers/theme-context.ts

@@ -0,0 +1,14 @@
+import { createContext } from 'react';
+
+type Theme = 'dark' | 'light' | 'system';
+
+interface ThemeContextType {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+  resolvedTheme: 'dark' | 'light';
+}
+
+const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
+
+export { ThemeContext };
+export type { Theme, ThemeContextType };