Phoenix 1.8 upgrade Tailwind issues

While upgrading an older phonix 1.7 app to the newer phoenix 1.8, I am running into an issue with the new tailwind css config

mix assets.build

13:45:27.039 [debug] Downloading tailwind from https://github.com/tailwindlabs/tailwindcss/releases/download/v4.1.7/tailwindcss-macos-x64

≈ tailwindcss v4.1.

Error: Can't resolve 'tailwindcss/base' in '/Users/satb/projects/myproj/assets/css'

I checked all places where I could. Can’t find which part of the upgrade I am missing.

mix.exs:

project section:

 compilers: [:phoenix_live_view] ++ Mix.compilers(),
 listeners: [Phoenix.CodeReloader]

deps section:

{:tailwind, "~> 0.3", runtime: Mix.env() == :dev},

aliases section:
"assets.deploy": [
        "tailwind myproj --minify",
        "esbuild myproj --minify",
        "phx.digest"
      ],

config.exs:

config :tailwind,
  version: "4.1.7",
  myproj: [
    args: ~w(
      --config=tailwind.config.js
      --input=assets/css/app.css
      --output=priv/static/assets/css/app.css
    ),
    cd: Path.expand("..", __DIR__)
  ]

I fixed the app.css file and copied the new version into it.

app.css at the top now:

@import "tailwindcss" source(none);
@source "../css";
@source "../js";
@source "../../lib/myproj_web";

Tried removing the tailwind.config.js file also and removing that option in mix.exs but made no difference.

What else is missing? Any help?