build(docker): move image to node 24 and install pnpm without corepack (#1265)

Supersedes #1250, whose 22 -> 26 bump failed the image build: `corepack enable`
exits 127 because Node 25 unbundled Corepack.

Both stages move to node:24-alpine, the current LTS line — Node 26 stays
Current until October 2026, which is the wrong target for a self-hosted runtime
image. pnpm is installed globally at the exact `packageManager` version, with
the `+sha512...` suffix stripped, so the next base-image major is a one-line
change instead of a broken build.

docs/architecture/pwa-self-hosted.md

@@ -175,7 +175,10 @@ browser sidecar data.
 The Docker image has two stages:
 
 1. Build stage installs dependencies and runs `web:pwa` plus `web-backend`.
-2. Runtime stage uses `node:22-alpine` with nginx installed. nginx serves
+   pnpm is installed globally at the exact `packageManager` version rather than
+   through Corepack, which Node 25 unbundled, so the base-image major stays
+   free to move.
+2. Runtime stage uses `node:24-alpine` with nginx installed. nginx serves
    `dist/apps/web` and proxies `/api/*` to the local Express backend.
    The entrypoint renders the nginx config from a `${PORT}` template, starts the
    backend, waits for `/health`, and then starts nginx. If either process exits