feat: add SPA, serve at /app/, update Dockerfile and README

- React + TanStack Router + shadcn/ui SPA under spa/
- serve spa/dist at /app/ with index.html fallback for client routing
- Dockerfile: node build stage for SPA, copy dist into runtime image
- README: document SPA, CORS_ORIGINS env var, architecture entry
- vite base set to /app/, manifest.json paths fixed

spa/src/components/theme-provider.tsx

@@ -0,0 +1,38 @@
+/* eslint-disable react-refresh/only-export-components */
+import * as React from "react"
+
+type ThemeProviderState = {
+  theme: "dark"
+  setTheme: (theme: "dark") => void
+}
+
+const ThemeProviderContext = React.createContext<
+  ThemeProviderState | undefined
+>(undefined)
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  React.useEffect(() => {
+    document.documentElement.classList.add("dark")
+  }, [])
+
+  const value = React.useMemo(
+    () => ({ theme: "dark" as const, setTheme: () => {} }),
+    [],
+  )
+
+  return (
+    <ThemeProviderContext.Provider value={value}>
+      {children}
+    </ThemeProviderContext.Provider>
+  )
+}
+
+export const useTheme = () => {
+  const context = React.useContext(ThemeProviderContext)
+
+  if (context === undefined) {
+    throw new Error("useTheme must be used within a ThemeProvider")
+  }
+
+  return context
+}