feat(ssa): Complete Phase V-A editable analysis plan variables

Features:
- Add editable variable selection in workflow plan (SingleVarSelect + MultiVarTags)
- Implement 3-layer flexible interception (warning bar + icon + blocking dialog)
- Add tool_param_constraints.json for 12 statistical tools parameter validation
- Add PATCH /workflow/:id/params API with Zod structural validation
- Implement synchronous parameter sync before execution (Promise chaining)
- Fix LLM hallucination by strict system prompt constraints
- Fix DynamicReport object-based rows compatibility (R baseline_table)
- Fix Word export row.map error with same normalization logic
- Restore inferGroupingVar for smart default variable selection
- Add ReactMarkdown rendering in SSAChatPane
- Update SSA module status document to v3.5

Modified files:
- backend: workflow.routes, ChatHandlerService, SystemPromptService, FlowTemplateService
- frontend: WorkflowTimeline, SSAWorkspacePane, DynamicReport, SSAChatPane, ssaStore, ssa.css
- config: tool_param_constraints.json (new)
- docs: SSA status doc, team review reports

Tested: Cohort study end-to-end execution + report export verified
Co-authored-by: Cursor <cursoragent@cursor.com>

frontend-v2/src/modules/ssa/index.tsx

@@ -9,6 +9,7 @@
 import React, { useEffect } from 'react';
 import { useSSAStore } from './stores/ssaStore';
 import SSAWorkspace from './SSAWorkspace';
+import './styles/ssa.css';
 import './styles/ssa-workspace.css';
 
 const SSAModule: React.FC = () => {