refactor(frontend): restructure project layout and update API schema

Relocate frontend source code from `next-app/` to `frontend/` to align with the new project structure. This includes removing the old Next.js boilerplate files and establishing a cleaner workspace.

Additionally, updates the OpenAPI specification to include support for the `immich` widget type and its corresponding configuration schema.

- Move frontend files to `frontend/`
- Delete obsolete `next-app/` directory and its configuration
- Add `immich` widget type to `openapi.yaml`
- Update `FrontendPlan.md` with dashboard refactor and UX direction

frontend/components/providers.tsx

@@ -0,0 +1,62 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import { QueryClientProvider } from "@tanstack/react-query";
+import { TooltipProvider } from "@/components/ui/tooltip";
+import { getQueryClient } from "@/lib/api/query-client";
+import { Theme, getStoredTheme, setStoredTheme, applyTheme } from "@/lib/theme/themes";
+
+export function Providers({ children }: { children: React.ReactNode }) {
+  const queryClient = getQueryClient();
+  const [theme, setTheme] = useState<Theme>("dark");
+  const [mswReady, setMswReady] = useState(false);
+
+  useEffect(() => {
+    const stored = getStoredTheme();
+    setTheme(stored);
+    applyTheme(stored);
+  }, []);
+
+  useEffect(() => {
+    if (process.env.NODE_ENV === "development" && process.env.NEXT_PUBLIC_API_BASE_URL === undefined) {
+      import("@/lib/mocks/browser").then(({ installMocks }) => {
+        installMocks();
+        setMswReady(true);
+      });
+    } else {
+      setMswReady(true);
+    }
+  }, []);
+
+  const changeTheme = (t: Theme) => {
+    setTheme(t);
+    setStoredTheme(t);
+    applyTheme(t);
+  };
+
+  if (!mswReady) {
+    return (
+      <div className="flex h-screen items-center justify-center bg-background text-foreground">
+        <span className="font-mono text-xs">[LOADING...]</span>
+      </div>
+    );
+  }
+
+  return (
+    <QueryClientProvider client={queryClient}>
+      <TooltipProvider delayDuration={300}>
+        <ThemeContext.Provider value={{ theme, setTheme: changeTheme }}>
+          {children}
+        </ThemeContext.Provider>
+      </TooltipProvider>
+    </QueryClientProvider>
+  );
+}
+
+import { createContext, useContext } from "react";
+
+type ThemeContextType = { theme: Theme; setTheme: (t: Theme) => void };
+export const ThemeContext = createContext<ThemeContextType>({ theme: "dark", setTheme: () => {} });
+export function useTheme() {
+  return useContext(ThemeContext);
+}