chore: upgrade Tailwind CSS v3 → v4

Upgrades Tailwind CSS and related packages:

- tailwindcss: ^3.4.19 → ^4.2.1
- prettier-plugin-tailwindcss: ^0.6.10 → ^0.7.2
- @tailwindcss/vite: new (replaces PostCSS Tailwind plugin for Vite)
- autoprefixer: removed (v4 handles vendor prefixes automatically)

Migration steps performed (using official @tailwindcss/upgrade tool + manual fixes):

1. src/app.css: Migrated from @tailwind directives to CSS-first approach:
   - @tailwind base/components/utilities → @import 'tailwindcss'
   - plugins (typography, forms, container-queries) → @plugin directives
   - content array → @source directive
   - Added v4 compatibility base styles for default border-color

2. vite.config.ts: Added @tailwindcss/vite plugin (recommended over PostCSS for
   Vite projects). Placed before sveltekit() for proper CSS processing order.

3. postcss.config.js: Removed tailwindcss and autoprefixer plugins (now handled
   by @tailwindcss/vite and v4's built-in vendor prefixing).

4. tailwind.config.ts: Deleted — theme was empty (no customisations) and
   plugins are now declared via @plugin in app.css.

No template changes needed: the project has no renamed utility usages
(shadow-sm, outline-none, ring, etc.) and no @apply in Sass files.
The Sass files (src/lib/scss/) are unaffected as they don't use Tailwind
directives.

All checks pass:
- svelte-check: 0 errors, 0 warnings
- build: success
- lint: success (Prettier + ESLint)

Closes #169
Part of epic #174

Author: josecelano