Add pre/post build stages, fix exec timeout, expand guest PATH

Build phases:
- Pre-build (apt update) and post-build (apt clean, autoremove, rm lists)
  run with 10-minute timeout; user recipe commands keep 30s timeout
- Log entries include phase field for UI grouping
- Always send explicit TimeoutSec to host agent (0 defaulted to 30s)

Frontend:
- Pre-build/post-build steps show phase label without exposing commands
- Recipe steps numbered independently starting from 1

Guest PATH:
- Add /usr/games:/usr/local/games to wrenn-init.sh PATH export
  (standard Ubuntu paths, needed for packages like cowsay)

frontend/src/lib/api/builds.ts

@@ -2,6 +2,7 @@ import { apiFetch, type ApiResult } from '$lib/api/client';
 
 export type BuildLogEntry = {
 	step: number;
+	phase: string; // "pre-build", "recipe", or "post-build"
 	cmd: string;
 	stdout: string;
 	stderr: string;