build: a clone compiles before the frontend is built

rust-embed refused to derive without `web/dist`, so on a fresh clone
`cargo check`, `cargo test` and rust-analyzer failed with a derive error that
did not say what was missing. With `allow_missing` the embed is empty until
the frontend is built, and the handler gives the answer it already had for
that case, "frontend not built". The release build is unchanged once
`web/dist` is there, which `make build`, the Dockerfile and CI all see to.

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: oven-sh/setup-bun@v2
-      # The binary embeds web/dist, so nothing compiles before the frontend is built.
+      # Built first so the binary under test embeds the app it ships with.
       - name: frontend
         working-directory: web
         run: |