fix(update): avoid killing in-place self-update

[setkyar]

Summary

• Fix in-app beta updates so pi install signals install.sh that pi-web is updating itself in place, avoiding service stop/restart from killing the npm install process.
• Add an in-place install path that atomically swaps the binary while the old process is still running, then lets pi-web restart itself afterward.
• Cover the updater command env and install script behavior, and wire the install script test into make test / make check.

Related issue

N/A

Type of change

• feat — new feature
• fix — bug fix
• docs — documentation only
• refactor — code change that neither fixes a bug nor adds a feature
• style — formatting / UI styling, no behavior change
• test — adding or updating tests
• chore — build, tooling, or maintenance

Live vs. Export

• Not applicable — this PR doesn't touch session rendering
• Considered both the live app and the export snapshot
• Kept internal/ui/live_templates/ in sync with web/src/session/ changes
• No live-only chrome (Vite scripts, active composer, SSE/API) leaked into export

Testing

• make check passes (test + build + vet)
• Frontend tests (vitest) cover the change
• Go tests (go test ./...) cover the change
• UI changes verified in a browser

Additional commands run:

• bash tests/install/inplace_test.sh
• go test ./internal/app