fix: resolve 7 frontend-backend API inconsistencies and add comprehensive documentation

- Fixed LoginResponse: added expires_in field
- Renamed format to file_format across FileInfo interface
- Updated ProcessRequest: replaced confidence_threshold with detect_layout
- Modified ProcessResponse: added message and total_files, removed task_id
- Removed non-existent getTaskStatus API call
- Fixed getOCRResult parameter from taskId to fileId
- Commented out translation config APIs pending backend implementation

Documentation:
- Added API_REFERENCE.md: Complete API endpoint inventory
- Added API_FIX_SUMMARY.md: Detailed before/after comparison of all fixes
- Added FRONTEND_API.md: Frontend integration documentation
- Added FRONTEND_QUICK_START.md: Quick start guide
- Added FRONTEND_UPGRADE_SUMMARY.md: Upgrade summary

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

frontend/src/types/api.ts

@@ -12,6 +12,7 @@ export interface LoginRequest {
 export interface LoginResponse {
   access_token: string
   token_type: string
+  expires_in: number  // Token expiration time in seconds
 }
 
 export interface User {
@@ -29,7 +30,7 @@ export interface FileInfo {
   id: number
   filename: string
   file_size: number
-  format: string
+  file_format: string  // Changed from 'format' to match backend
   status: 'pending' | 'processing' | 'completed' | 'failed'
 }
 
@@ -37,13 +38,15 @@ export interface FileInfo {
 export interface ProcessRequest {
   batch_id: number
   lang?: string
-  confidence_threshold?: number
+  detect_layout?: boolean  // Changed from confidence_threshold to match backend
 }
 
 export interface ProcessResponse {
-  task_id: string
+  message: string       // Added to match backend
   batch_id: number
+  total_files: number   // Added to match backend
   status: string
+  // Removed task_id - backend uses batch-level tracking instead
 }
 
 export interface TaskStatus {
@@ -132,7 +135,7 @@ export interface ExportRule {
 export interface CSSTemplate {
   name: string
   description: string
-  filename: string
+  // filename is not returned by backend - use name as identifier
 }
 
 // Translation (FUTURE FEATURE)