Package beans-serve (web UI) from source via nix (flakes-qe0o)

The nix package added in flakes-6ne0 installs the prebuilt beans release binary (CLI + beans tui). The public release does not publish beans-serve (the web UI / GraphQL server, ~59MB because it embeds the SvelteKit frontend), so the bw = beans-serve fish abbr still relies on the imperative ~/.local/bin/beans-serve.

To package beans-serve declaratively requires building from source (github.com/hmans/beans), which is a two-stage build:

1. Frontend: frontend/ is SvelteKit + pnpm. pnpm install + pnpm build, output copied into internal/web/dist/ (consumed by //go:embed dist/* in internal/web/embed.go). Needs a pnpmDeps fixed-output hash.
2. Backend: go build ./cmd/beans-serve (and optionally ./cmd/beans, ./cmd/beans-tui) via buildGoModule with a vendorHash. Note internal/graph/generated.go appears committed, but mise.toml’s build runs go generate ./... (gqlgen) first — confirm whether codegen is needed or the committed output suffices.

Open questions

• Is the web UI (beans-serve) actually still used, or has the workflow moved to TUI / external beans server? If unused, scrap this.
• Prefer building all three from source (replacing the prebuilt-binary package in flakes-6ne0 for consistency) vs. keeping prebuilt beans + source-built beans-serve?

Tasks (if pursued)

• Frontend derivation (pnpm) -> dist
• buildGoModule with vendorHash, embedding the built frontend
• Wire into home.packages; update bw/bt abbrs as needed; drop imperative ~/.local/bin/beans*