Replace submodules and symlinks with apps.json registry

[nickustinov]

Summary

• Replace git submodules, symlinks, and per-app adapter files with a single apps.json registry
• External apps are now served as standalone web apps through their own index.html – no more AppModule wrapping
• APP_PATH=../my-app ./start-even.sh works for one-off testing without editing apps.json
• Extract Vite plugins into vite-plugins/ directory for app-specific server-side config
• Add app-server plugin that auto-starts an app's backend server if it has a server/ directory
• Rewrite README with updated architecture, app building guide, and developer tips

What changed

Removed:

• .gitmodules and all submodule references (chess, epub, reddit, stars, transit, weather, misc/editor)
• All src/*-submodule-adapter.ts files
• preserveSymlinks config

Added:

• apps.json – maps app names to git URLs (auto-cloned into .apps-cache/) or local paths
• vite-plugins/ – extracted and new plugins (app-server, browser-launcher, chess-stockfish, reddit-proxy, restapi-proxy)
• APP_PATH support in start-even.sh for pointing to any local app directory

Modified:

• vite.config.ts – serves external app's own index.html via middleware, reads apps.json for aliases and fs.allow
• src/Main.ts – simplified to only handle built-in apps
• start-even.sh – reads apps from apps.json, supports APP_PATH shortcut, auto-installs app deps
• README.md – rewritten for v0.0.4

How it works

• Built-in apps (demo, clock, timer, etc.) work exactly as before via apps/ directory
• External apps registered in apps.json are cloned on first run and served directly – their own index.html is intercepted and rewritten with /@fs/ paths so Vite resolves everything from the app's directory
• APP_PATH lets developers point to any local checkout without touching apps.json

Test plan

• ./start-even.sh – built-in app selection works (demo, clock, timer)
• ./start-even.sh – external apps from apps.json are cloned and launched
• APP_PATH=../my-app ./start-even.sh – local app served correctly
• Apps with server/ directory get their backend auto-started