refactor(sveltekit-embed): drop Skeleton UI in favour of plain Tailwind v4

[ReneWerner87]

Stacked on #5299. Base is claude/sveltekit-template-build-fix, since this needs the @sveltejs/vite-plugin-svelte and cookie fixes from that PR. Merging #5299 first retargets this to master automatically.

Completes the last broken template. Follows the direction you picked: remove Skeleton, keep plain Tailwind.

Why removing rather than migrating

sveltekit-embed has not built since @skeletonlabs/skeleton was bumped 2 to 5. v5 removed AppShell, AppBar, LightSwitch, CodeBlock and storeHighlightJs, along with the tailwind/skeleton.cjs and styles/skeleton.css entry points this recipe imported. The package now exports only ., ./package.json and ./themes/*.

Every component the layout used has to be hand-written either way, so the dependency was no longer paying for itself. This recipe exists to show Fiber serving an embedded SvelteKit build, not to demonstrate a component library.

Removing it also takes @tailwindcss/forms, @tailwindcss/typography (pulled in only for Skeleton) and @floating-ui/dom (a Skeleton peer) out of the tree, four fewer dependencies to break on the next major.

Replacements

Two small components in src/lib:

• CodeBlock.svelte renders highlight.js output with an optional line-number gutter, keeping the same lineNumbers / language / code / rounded props the call sites already passed, so +page.svelte needed no prop changes.
• LightSwitch.svelte toggles a dark class on the root element, remembers the choice in localStorage, falls back to prefers-color-scheme.

The Skeleton utility classes the markup relied on (h1, h3, blockquote, card, btn variant-outline-primary) are replaced with plain Tailwind. The primary palette Skeleton's theme provided is declared in an @theme block, so bg-primary-200, text-primary-900 and friends keep working unchanged.

Also in here

• Tailwind v4 migration, the same one entgo-sveltekit got in fix(sveltekit): repair entgo-sveltekit build and bound the cookie override #5299: the @tailwindcss/vite plugin replaces the v3-style PostCSS setup. postcss.config.cjs, tailwind.config.cjs, postcss and autoprefixer are removed as redundant under v4.
• src/app.postcss renamed to src/app.css. The .postcss extension made Vite route the file through postcss-import, which tries to resolve @import "tailwindcss" as a file path and dies with ENOENT. The Vite plugin only claims .css. This is why entgo-sveltekit worked with the identical config and this template did not.
• Fixed the two self-closing <span /> tags Svelte 5 warns about, since the file was being touched anyway.

Verification

• pnpm run build completes, the template builds for the first time in a while
• pnpm install --frozen-lockfile passes
• OSV scan of the refreshed lockfile: clean
• no skeletonlabs references left outside the lockfile

Visually close to the original but not pixel-identical. The header, footer button and browser mockup are rebuilt with Tailwind equivalents rather than Skeleton's component styling.