Surface LLM partial failures as user-visible warnings in scan results

[Copilot]

LLM analysis nodes (summary_generation_node, impact_analysis_node, privacy_compliance_node) silently swallow exceptions and set their outputs to None, leaving scan results with empty sections and no explanation to the user.

Backend

• WorkflowState — adds llm_warnings: Optional[list] field to carry accumulated failure messages through the pipeline
• nodes.py — each LLM node reads the current warnings list, appends a message if its output is None, and writes the updated list back to state:

  warnings = list(state.get("llm_warnings") or [])
  if executive_summary is None:
      warnings.append("Summary unavailable — LLM service temporarily failed")
  return Command(goto=..., update={"executive_summary": executive_summary, "llm_warnings": warnings})

• api/main.py — exposes partial_failures in the scan result response (empty list on full success, warning strings on partial failure)

Frontend

• SummaryPanel.jsx — reads rawScanResult.partial_failures and renders an amber warning banner above summary content in every display branch; accessible via role="alert" with a descriptive aria-label
• SummaryPanel.scss — styles for the banner with dark- and light-theme variants

Tests

• 9 new unit tests in tests/workflow/test_nodes_llm_warnings.py covering: no warning on success, generic exception, LLMFallbackError, and multi-node accumulation across all three LLM nodes