fix(desktop): add HTML artifact Finder fallback

[Dante-dan]

Summary

• Add the existing localized Show in Finder action to the HTML artifact read-failure banner while leaving successful sandboxed previews unchanged.
• Reuse the Artifact Pane's existing openPath(sessionId, artifactId) route, so materialization and path validation remain in the main process.
• Add a focused rendered-failure regression and strengthen the desktop service adapter assertion.

PR #5216 independently proposed wiring the same failure-state callback. This version keeps the issue's failure-only scope and adds regression coverage without changing successful HTML artifact opening.

Fixes #5215

Verification

• Desktop workspace dependency build passed.
• Desktop build:main, typecheck, and renderer production build passed.
• 27 focused HTML fallback, adapter, artifact IPC, open-path guard, and external-link guard tests passed.
• Biome passed on all four changed files.
• git diff --check origin/main...HEAD passed.
• Independent acceptance reran the main-process TypeScript compile and three focused suites: 15 passed, 0 failed.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex implemented and tested the focused UI fallback; a separate Codex acceptance pass reviewed the diff and reran targeted validation.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No