Add make build to embed the built PWA in the server binary

Fixes review finding F1: nothing copied web/dist into
internal/frontend/dist, so the server always embedded the placeholder.
make build now builds the web app, replaces internal/frontend/dist with
web/dist, and builds bin/mealquest.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -8,6 +8,7 @@ deploy/secrets/
 # Build artifacts and dependencies
 node_modules
 dist
+bin/
 # Placeholder so the Go embed compiles on a fresh clone; built assets stay ignored.
 !internal/frontend/dist/
 internal/frontend/dist/*