feat(frontend): migrate to Tailwind CSS v4 (#852, supersedes #686)

[hellodk]

Summary

• Upgrades tailwindcss from v3.4.x to v4.3.1 and replaces the PostCSS plugin with the dedicated @tailwindcss/vite Vite plugin for better DX and faster builds
• Removes autoprefixer and postcss dev-dependencies (v4 handles vendor prefixing automatically)
• Migrates the JS config (tailwind.config.js) to a CSS-first @theme {} block inside src/index.css, preserving the full brand color palette
• Renames all deprecated/renamed utility classes across the entire frontend/src/ tree per the official v4 upgrade guide

Plugin approach

@tailwindcss/vite — the Vite-native plugin. This is the recommended approach for Vite projects; it integrates directly into Vite's transform pipeline and requires no PostCSS configuration.

Theme approach

CSS-first @theme {} — the brand color tokens are now declared as CSS custom properties directly in src/index.css:

@import "tailwindcss";

@theme {
  --color-brand-50: #f0f4ff;
  /* … */
  --color-brand-950: #1f2064;
}

Utility classes renamed

v3 class	v4 class	Count
shadow-sm	shadow-xs	106
outline-none	outline-hidden	114
flex-shrink-0	shrink-0	114

Verification

• pnpm build (tsc -b && vite build): passed — 71 KB CSS bundle generated
• pnpm lint: passed — 0 errors, 2 pre-existing warnings (unrelated to this PR)
• pnpm test: passed — 5/5 tests

Closes #852

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[github-actions]

Coverage Report

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✅ 85.9% on fleet_platform/services/ (gate: 80%)

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[github-actions]

Coverage Report

✅ 85.9% on fleet_platform/services/ (gate: 80%)

Gate passed