style(project): format Go with gofmt, set up Prettier for the frontend

Run gofmt -w across the backend, normalising the manually-aligned := blocks
to the gofmt standard. No code behaviour changes.

Add Prettier (+ prettier-plugin-svelte) to the frontend with the SvelteKit
default config (tabs, single quotes) so formatting is reproducible, then run
it over the whole tree. Add format / format:check npm scripts and a
.prettierignore (build output, generated schema.ts, static assets).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/service/user_service.go

@@ -189,4 +189,4 @@ func (s *UserService) Delete(ctx context.Context, id int16) error {
 	}
 	_ = s.audit.Log(ctx, "user_delete", nil, nil, map[string]any{"target_user_id": id})
 	return nil
-}
+}