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_dev.go

@@ -0,0 +1,9 @@
+// assets_dev.go — used in non-production builds (no web/build required)
+//go:build !production
+
+package assets
+
+import "io/fs"
+
+// FS is nil in dev mode; the router skips static file serving.
+var FS fs.FS