fix: move embed to repo root; make build sequential

- Move embed_prod.go/embed_dev.go out of cmd/wotra/ into root-level
  assets_prod.go / assets_dev.go (package assets). go:embed paths
  must be relative to the source file; cmd/wotra/ cannot reach
  ../../web/build.
- Update cmd/wotra/main.go to import the root assets package.
- Change mise.toml build task from parallel depends to sequential
  shell commands (build:web then build:go) so the frontend is always
  compiled before the Go embed runs.

assets_prod.go

@@ -0,0 +1,21 @@
+// assets_prod.go — used in production builds (requires web/build to exist)
+//go:build production
+
+package assets
+
+import (
+	"embed"
+	"io/fs"
+)
+
+//go:embed all:web/build
+var embeddedWeb embed.FS
+
+// FS is the sub-filesystem rooted at web/build.
+var FS fs.FS = func() fs.FS {
+	sub, err := fs.Sub(embeddedWeb, "web/build")
+	if err != nil {
+		panic(err)
+	}
+	return sub
+}()