[GUI] selectedArtifact fallback causes unexpected selection jump

[Vantalens]

File: packages/gui/src/App.tsx:68

Severity: Medium

Description:
selectedArtifact fallback to artifacts[0] when selectedArtifactId not found causes selection to jump unexpectedly when artifact list updates.

Code:

const selectedArtifact = artifacts.find((artifact) => artifact.id === selectedArtifactId) || artifacts[0];

Failure Scenario:

• User selects artifact with id='artifact-5'
• Backend updates artifacts array (new artifact added at beginning or order changed)
• selectedArtifactId becomes stale (old artifact removed)
• useMemo returns artifacts[0] instead of null/undefined
• UI jumps to different artifact without user action
• User sees wrong artifact preview, confusion ensues

Suggested Fix:
Return null/undefined when selectedArtifactId not found, show empty state or keep last valid selection until user explicitly chooses new one.