Starting transition to Svelte 5 + Tailwind CSS 4.

- Upgraded all dependencies\* to latest versions.
- Using Legacy mode in Svelte 5. Svelte code upgrade TKTK.
- Upgraded Tailwind CSS to v4. Small visual discrepancies exist with the
current Tailwind CSS v3 deployment. Report in #1203.

\*`flowbite-svelte` current version is `v1.3.0` and introduces a lot of
breaking changes from our current `v0.47.3` version. We'll be switching
to `shadcn-svelte` soon so we can integrate `svelte-ai-elements` later
on. Therefore, there's no reason to upgrade and fix breaking changes.
`flowbite-svelte` introduced support for TailwindCSS 4 in `v0.48.5`, so
we're upgrading to that version.