fix: serve SvelteKit _app assets correctly

- Use all: prefix in go:embed to include _app/ directory (Go skips
  _-prefixed dirs by default)
- Move jsonContentType middleware to /api route group only
- Use http.ServeContent for proper MIME type detection
- Remove unused fileServer variable

web.go

@@ -5,5 +5,5 @@ import "embed"
 // WebBuildFS holds the embedded SvelteKit static build output.
 // The build directory is populated by running `npm run build` in the web/ directory.
 //
-//go:embed web/build
+//go:embed all:web/build
 var WebBuildFS embed.FS