feat: structured artifact rendering, UX improvements

- Render structured artifacts from agent results with type-aware
  formatting: code blocks with syntax highlighting and copy button,
  terminal-style command output, search result cards, and text findings
- Make FinalResult panel collapsible (default collapsed) with scrollable
  content (max-h-96) to prevent dominating the chat view
- Add clickable URL detection in summaries and artifact content
- Fix code block contrast for both light and dark mode
- Animate progress bar with pulse ring on active step and gradient
  shimmer on connecting lines
- Fix tab-switching bug: use module-level orchestrationStore singleton
  so orchestration state survives route navigation
- Remove sample/demo data seeding and clean up persisted localStorage
  entries from previous sample sessions
- Remove showSampleBadge prop from PageHeader
- Regenerate proto types for new Artifact message and ArtifactType enum
- Update README project structure (remove deleted data/ directory)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -84,7 +84,6 @@ src/
   lib/
     components/      Svelte components (Backdrop, PageHeader, MessageInput, ...)
     composables/     Reactive composables (useOrchestration)
-    data/            Sample/demo data generators
     proto/           Generated protobuf TypeScript
     services/        gRPC client (orchestrator)
     stores/          Svelte 5 reactive stores (sessions, audit, memory, theme, ...)