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/lib/theme/themes.ts

@@ -0,0 +1,26 @@
+export type Theme = "light" | "dark" | "casaos";
+
+const STORAGE_KEY = "dash-theme";
+
+export function getStoredTheme(): Theme {
+  if (typeof window === "undefined") return "dark";
+  const stored = localStorage.getItem(STORAGE_KEY);
+  if (stored === "light" || stored === "dark" || stored === "casaos") return stored;
+  return "dark";
+}
+
+export function setStoredTheme(theme: Theme) {
+  if (typeof window === "undefined") return;
+  localStorage.setItem(STORAGE_KEY, theme);
+}
+
+export function applyTheme(theme: Theme) {
+  if (typeof document === "undefined") return;
+  document.documentElement.setAttribute("data-theme", theme);
+}
+
+export const themeLabels: Record<Theme, string> = {
+  light: "Light",
+  dark: "Dark",
+  casaos: "CasaOS",
+};