[Fix] Environment boot failures show no useful error feedback

[roomote-roomote]

Opened on behalf of Matt Rubens. Follow up by mentioning @roomote-roomote, in the web UI, or in Discord.

Related issue

Fixes #538

Why this PR exists

• A maintainer explicitly invited this PR in the linked issue or discussion
• I am a maintainer / this is internal Roomote work

What changed

When an environment fails while booting, the startup UI used to leave people on Booting environment with little context, then fail with a raw docker run … command dump instead of the actual cause.

This PR:

• Prefer Docker daemon stderr when spawn fails so causes like missing images / pull denied reach the run error field
• Rewrite common worker-boot failures into actionable UI copy (missing worker image, container exited early, fetch failed, generic Docker command dumps)
• Show Still booting… (elapsed: …) after 15s on the active startup step so long boots are visibly alive

How it was tested

• Vitest: controller (BaseController, docker-sandbox-security) and web (task-run-errors, StartupMessage.client)
• Typecheck: @roomote/controller and @roomote/web
• Storybook screenshots of the failed missing-image and still-booting states

Checklist

• The PR title follows the repo convention: [Fix], [Feat], [Improve], [Refactor], [Docs], or [Chore] followed by a user-facing description
• This PR is small and scoped to one change
• pnpm lint and pnpm check-types pass locally
• I added tests or included a clear manual validation note above
• I removed secrets, tokens, private keys, and customer data from code, logs, and screenshots
• If this change should appear in the changelog, I ran pnpm changeset

Screenshots

[roomote-roomote]

No code issues found. See task

_{Reviewed af513ac}