chore: migrate Tailwind CSS v3 to v4 (PostCSS plugin split)

[Diyaaa-12]

Summary

Migrates apps/web from Tailwind CSS v3 to v4, fixing the PostCSS plugin split that was blocking Dependabot PR #412.

• Removed tailwindcss v3 + autoprefixer, installed tailwindcss 4.3.3 + @tailwindcss/postcss
• Updated postcss.config.cjs to use @tailwindcss/postcss instead of the old tailwindcss/autoprefixer plugin pair
• Replaced the three @tailwind directives in globals.css with the v4 @import "tailwindcss"; syntax
• Removed tailwind.config.js — theme.extend was empty and there were no custom plugins, so v4's automatic content detection covers this without a config file
• Unrelated pre-existing peer dependency warning in apps/docs (vitepress → @docsearch/react wanting React <19) — not in scope for this PR
• Once merged, dependabot PR chore(deps): bump tailwindcss from 3.4.19 to 4.3.3 #412 can be closed/superseded

Test plan

• pnpm --filter @meridian/web build passes (tsc + vite build clean)
• Full Playwright E2E suite passes (12/12)
• Manually checked the rendered app for v4 default changes (border color, ring width) — no visual regressions observed

Closes #445

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