feat: implement 5 QA-driven security and quality proposals

Implemented proposals from comprehensive QA review:

1. extend-csrf-protection
   - Add POST to CSRF protected methods in frontend
   - Global CSRF middleware for all state-changing operations
   - Update tests with CSRF token fixtures

2. tighten-cors-websocket-security
   - Replace wildcard CORS with explicit method/header lists
   - Disable query parameter auth in production (code 4002)
   - Add per-user WebSocket connection limit (max 5, code 4005)

3. shorten-jwt-expiry
   - Reduce JWT expiry from 7 days to 60 minutes
   - Add refresh token support with 7-day expiry
   - Implement token rotation on refresh
   - Frontend auto-refresh when token near expiry (<5 min)

4. fix-frontend-quality
   - Add React.lazy() code splitting for all pages
   - Fix useCallback dependency arrays (Dashboard, Comments)
   - Add localStorage data validation in AuthContext
   - Complete i18n for AttachmentUpload component

5. enhance-backend-validation
   - Add SecurityAuditMiddleware for access denied logging
   - Add ErrorSanitizerMiddleware for production error messages
   - Protect /health/detailed with admin authentication
   - Add input length validation (comment 5000, desc 10000)

All 521 backend tests passing. Frontend builds successfully.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

openspec/specs/dashboard/spec.md

@@ -161,6 +161,33 @@ The system SHALL support project templates to standardize project creation.
 - **THEN** system creates template with project's CustomField definitions
 - **THEN** template is available for future project creation
 
+### Requirement: Code Splitting
+The application SHALL use code splitting with React.lazy() to reduce initial bundle size and improve load times.
+
+#### Scenario: Initial page load
+- **WHEN** user navigates to application
+- **THEN** only core framework and current route are loaded
+- **AND** other routes are loaded on demand
+
+#### Scenario: Route-based splitting
+- **WHEN** user navigates to a different page
+- **THEN** that page's code chunk is loaded dynamically
+- **AND** loading fallback is displayed during load
+
+### Requirement: LocalStorage Data Validation
+User data loaded from localStorage SHALL be validated before use to prevent crashes from corrupted data.
+
+#### Scenario: Corrupted localStorage data
+- **WHEN** localStorage contains malformed user JSON
+- **THEN** invalid data is cleared
+- **AND** user is redirected to login page
+- **AND** no application crash occurs
+
+#### Scenario: Valid localStorage data
+- **WHEN** localStorage contains valid user JSON
+- **THEN** user is authenticated from stored data
+- **AND** application loads normally
+
 ### Requirement: Error Boundary Protection
 The frontend application SHALL gracefully handle component render errors without crashing the entire application.