chore: upgrade Vite v6 → v7 and @sveltejs/vite-plugin-svelte v5 → v6

Upgrades:
- vite: ^6.4.1 → ^7.3.1
- @sveltejs/vite-plugin-svelte: ^5.1.1 → ^6.2.4

Both packages are upgraded together since vite-plugin-svelte v6 requires
Vite ^6.3.0 || ^7.0.0.

Breaking changes in Vite v7 that were assessed:
- Node.js 18 dropped (we use Node 22 — no impact)
- Default browser target updated to baseline-widely-available (aligns well
  with Tailwind v4 which also targets modern browsers)
- Sass legacy API removed — no impact, vite.config.ts has no
  css.preprocessorOptions for sass/scss
- splitVendorChunkPlugin removed — not used in this project
- All other advanced API removals are not used in this project

No changes to vite.config.ts were needed.

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

Closes #170
Closes #173
Part of epic #174

Author: josecelano