Merge #180: chore: upgrade Tailwind CSS v3 → v4

738d545 chore: upgrade Tailwind CSS v3 → v4 (Jose Celano)

Pull request description:

  ## Overview

  Upgrades Tailwind CSS from v3 to v4 — a ground-up rewrite with a CSS-first configuration model.

  Part of epic #174.

  Closes #169

  ## Package Changes

  | Package | From | To |
  |---------|------|----|
  | `tailwindcss` | `^3.4.19` | `^4.2.1` |
  | `prettier-plugin-tailwindcss` | `^0.6.10` | `^0.7.2` |
  | `@tailwindcss/vite` | — | `^4.2.1` (new) |
  | `autoprefixer` | `^10.4.24` | **removed** (v4 handles this automatically) |

  The plugin packages (`@tailwindcss/typography`, `@tailwindcss/forms`, `@tailwindcss/container-queries`) stay at their current versions — they work with v4 via the new `@plugin` directive.

  ## Migration Details

  Used the official `@tailwindcss/upgrade` tool for the initial migration, then applied manual fixes.

  ### [src/app.css](src/app.css)
  - `@tailwind base/components/utilities` → `@import 'tailwindcss'`
  - Plugins moved from `tailwind.config.ts` → `@plugin` directives
  - `content` array migrated to `@source` directive
  - Added v4 compatibility base styles for default `border-color` (was `gray-200`, now `currentColor`)

  ### [vite.config.ts](vite.config.ts)
  - Added `@tailwindcss/vite` plugin (recommended for Vite projects over PostCSS)
  - Placed before `sveltekit()` for correct CSS processing order

  ### [postcss.config.js](postcss.config.js)
  - Removed `tailwindcss` and `autoprefixer` plugins (both now handled internally by `@tailwindcss/vite`)

  ### tailwind.config.ts — **deleted**
  - Theme had no customizations (empty `extend: {}`)
  - Plugins are now declared via `@plugin` in `app.css`

  ## What Was NOT Needed

  - **No template changes**: the codebase doesn't use any renamed utilities (`shadow-sm` → `shadow-xs`, `outline-none` → `outline-hidden`, `ring` → `ring-3`, etc.)
  - **No Sass changes**: `src/lib/scss/` files and `<style lang="scss">` component blocks are unaffected — they use pure Sass features and no `@apply`

  ## Testing Checklist

  - [x] `svelte-check`: 0 errors, 0 warnings
  - [x] `npm run build`: success
  - [x] `npm run lint`: success (Prettier + ESLint)
  - [x] Pre-commit hooks pass
  - [ ] Visual review in browser — all pages render correctly
  - [ ] Typography styles apply correctly (prose classes)
  - [ ] Form styles work
  - [ ] Container queries work
  - [ ] Responsive layouts work

ACKs for top commit:
  josecelano:
    ACK 738d545

Tree-SHA512: 598bb18cb71e76cbac3d1961b94e7e2ade212f30b7590f0c4a69d7dc6e8f3fb21db33e3d26b6cac11e2e2730b76ee5280b60d457f0a14dfbb4c875a38445a5b6

Author: josecelano