fix(web): run Tailwind 4 through @tailwindcss/vite, not PostCSS (#5021)

The image build on fecd030b failed in `vite build`:

  [postcss] ENOENT: no such file or directory, open '/web/tailwindcss'

With a PostCSS config present, Vite's own @import inliner resolves
`@import 'tailwindcss'` before @tailwindcss/postcss sees it, and reads it
as a relative file. svelte-check and Vitest never build CSS, so only the
image job caught it. The Vite plugin is Tailwind's documented setup for
Vite projects and handles the import itself, so postcss.config.cjs and
the direct postcss and @tailwindcss/postcss dependencies go.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/package-lock.json

@@ -16,12 +16,11 @@
         "@sveltejs/adapter-static": "^4.0.0",
         "@sveltejs/kit": "^3.0.0",
         "@sveltejs/vite-plugin-svelte": "^7.0.0",
-        "@tailwindcss/postcss": "^4.3.3",
+        "@tailwindcss/vite": "^4.3.3",
         "@testing-library/jest-dom": "^6.9.1",
         "@testing-library/svelte": "^5.3.1",
         "@types/node": "^25.9.1",
         "jsdom": "^25.0.1",
-        "postcss": "^8.5.15",
         "svelte": "^5.57.1",
         "svelte-check": "^4.7.5",
         "tailwindcss": "^4.3.3",
@@ -37,19 +36,6 @@
       "dev": true,
       "license": "MIT"
     },
-    "node_modules/@alloc/quick-lru": {
-      "version": "5.2.0",
-      "resolved": "https://registry.npmjs.org/@alloc/quick-lru/-/quick-lru-5.2.0.tgz",
-      "integrity": "sha512-UrcABB+4bUrFABwbluTIBErXwvbsU/V7TZWfmbgJfbkwiBuziS9gxdODUyuiecfdGQ85jglMW6juS3+z5TsKLw==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=10"
-      },
-      "funding": {
-        "url": "https://github.com/sponsors/sindresorhus"
-      }
-    },
     "node_modules/@asamuzakjp/css-color": {
       "version": "3.2.0",
       "resolved": "https://registry.npmjs.org/@asamuzakjp/css-color/-/css-color-3.2.0.tgz",
@@ -1639,17 +1625,18 @@
         "node": ">= 20"
       }
     },
-    "node_modules/@tailwindcss/postcss": {
+    "node_modules/@tailwindcss/vite": {
       "version": "4.3.3",
-      "resolved": "https://registry.npmjs.org/@tailwindcss/postcss/-/postcss-4.3.3.tgz",
-      "integrity": "sha512-JTSZZGQi1AyKirbLN3azmjVzef92tcX7h+iSqPdaeStyFpGpDlKvvpxeOE8njhbUanbRwr3z8DyzhICWnMtQeg==",
+      "resolved": "https://registry.npmjs.org/@tailwindcss/vite/-/vite-4.3.3.tgz",
+      "integrity": "sha512-yYU8cogLeSh/ms2jh8Fj7jaba/EWa7Ja6GoUqYZaraEuCI5YS6ms6ObZgjjedm+jm6XZjdNRWBpPP6Z86oOxcw==",
       "dev": true,
       "dependencies": {
-        "@alloc/quick-lru": "^5.2.0",
         "@tailwindcss/node": "4.3.3",
         "@tailwindcss/oxide": "4.3.3",
-        "postcss": "^8.5.16",
         "tailwindcss": "4.3.3"
+      },
+      "peerDependencies": {
+        "vite": "^5.2.0 || ^6 || ^7 || ^8"
       }
     },
     "node_modules/@tanstack/query-core": {

web/package.json

@@ -15,12 +15,11 @@
     "@sveltejs/adapter-static": "^4.0.0",
     "@sveltejs/kit": "^3.0.0",
     "@sveltejs/vite-plugin-svelte": "^7.0.0",
-    "@tailwindcss/postcss": "^4.3.3",
+    "@tailwindcss/vite": "^4.3.3",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/svelte": "^5.3.1",
     "@types/node": "^25.9.1",
     "jsdom": "^25.0.1",
-    "postcss": "^8.5.15",
     "svelte": "^5.57.1",
     "svelte-check": "^4.7.5",
     "tailwindcss": "^4.3.3",

web/postcss.config.cjs

@@ -1,5 +0,0 @@
-module.exports = {
-  plugins: {
-    '@tailwindcss/postcss': {},
-  }
-};

web/vite.config.ts

@@ -1,6 +1,7 @@
 import adapter from '@sveltejs/adapter-static';
 import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
 import { sveltekit } from '@sveltejs/kit/vite';
+import tailwindcss from '@tailwindcss/vite';
 import { defineConfig } from 'vite';
 import { execSync } from 'node:child_process';
 import tokens from './src/lib/styles/tokens.json' with { type: 'json' };
@@ -76,7 +77,7 @@ export const kitOptions = {
 };
 
 export default defineConfig({
-  plugins: [tokensPlugin, sveltekit(kitOptions), injectGoTokensPlugin],
+  plugins: [tailwindcss(), tokensPlugin, sveltekit(kitOptions), injectGoTokensPlugin],
   server: {
     port: 5173,
     proxy: {