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.

mise.toml

@@ -10,8 +10,11 @@ DB_PATH = "wotra.db"
 TZ = "UTC"
 
 [tasks.build]
-description = "Build production single binary (web + Go)"
-depends = ["build:web", "build:go"]
+description = "Build production single binary (web + Go, sequential)"
+run = """
+mise run build:web
+mise run build:go
+"""
 
 [tasks."build:web"]
 description = "Build Svelte frontend"