test: automate desktop protocol journey

[oratis]

Summary

• turn preview-app.html into a protocol-aware Tauri fixture using the production renderer bridge
• add Playwright coverage for shell geometry, session resume, fast-turn buffering, approval/tool/usage completion, and Files source/diff/history
• keep completed turns mounted while refreshing the sidebar and prevent tool results from rewriting older resumed turns
• add a dedicated desktop browser journey CI job with failure artifacts

Validation

• pnpm install --frozen-lockfile
• pnpm typecheck
• pnpm lint (0 errors; one pre-existing warning)
• pnpm format:check
• pnpm docs:check
• pnpm test (953 passed, 12 skipped)
• pnpm build
• pnpm --filter @deepcode/desktop test:e2e (3 passed)

Stack

Depends on #192.

[oratis]

Review: approved ✅

Real browser-level coverage for the desktop client, using the production renderer bridge rather than a hand-written mock. That distinction is what makes the fixture worth maintaining — a mock would drift from the bridge and pass while the real thing broke.

What I checked

• Coverage spans the flows that actually regressed historically: shell geometry, session resume, fast-turn buffering, approval/tool/usage completion, and Files source/diff/history.
• "Keep completed turns mounted while refreshing the sidebar" and "prevent tool results from rewriting older resumed turns" are both fixes, not just tests. The second is a genuine correctness bug — a late tool result landing on a resumed historical turn would visibly rewrite history in the UI.
• Dedicated CI job with failure artifacts. Browser-journey failures are near-impossible to debug from a log line alone; screenshots/traces on failure are what keep this from becoming a flaky job people skip.

Maintenance note: this is the stack's only end-to-end UI gate, so it is also the one most likely to go flaky and get muted. Worth watching its first few weeks — a muted journey test is worse than none, because it still looks like coverage.

Validation: CI green including the new desktop journey job.