feat: UmbrelOS-style CE with iStore, Casdoor auth, and App Manager

- iStore service (port 3021): fetches app manifests from git.falahos.my/falahos
  org via Gitea API, 5-min cache, query/search support
- UmmahID login: replaced stub form with Casdoor OAuth2 PKCE flow (RFC 7636),
  no client secret required; /api/auth/config and /api/auth/token endpoints
  added to server.cjs
- App Manager service (port 3022): installs/uninstalls Docker apps via
  /var/run/docker.sock, pulls images, starts containers on falah-net,
  persists state to /data/installed.json
- docker-compose.yml: orchestrates falah-app + istore + app-manager with
  health checks, named volumes, and shared falah-net network
- install.sh: one-line curl installer — checks prereqs, clones repo,
  prompts for credentials, builds and starts all containers
- .env.example: all env vars documented (CASDOOR_CLIENT_ID, GITEA_TOKEN,
  APP_MANAGER_PORT, etc.)
- Frontend: IStore overlay now fetches live apps from Gitea, real
  install/uninstall via App Manager with 2s polling, indeterminate
  progress bar, Uninstall button for installed apps
- app.yml.example: developer manifest spec for publishing to iStore

services/istore/Dockerfile

@@ -0,0 +1,11 @@
+FROM node:20-alpine
+WORKDIR /app
+
+COPY package.json ./
+RUN npm install --production
+
+COPY server.cjs ./
+
+EXPOSE 3021
+ENV NODE_ENV=production
+CMD ["node", "server.cjs"]