The application package you're looking for is a single HTML file with no external dependencies, especially if it avoids minification, obfuscation, or the use of technologies like WASM blobs which require complex external toolchains to disassemble or modify. This is very achievable right now! See tools like TiddlyWiki or Decker, for example.

The primary barrier to an entirely server-free or server-agnostic webapp ecosystem is browser vendors choosing to seal the newest JavaScript APIs behind "secure contexts" which are only available to documents served over HTTPS.