feat: add batch processing for multiple file uploads

- Add BatchState management in taskStore with progress tracking
- Implement batch processing service with concurrency control
  - Direct Track: max 5 parallel tasks
  - OCR Track: sequential processing (GPU VRAM limit)
- Refactor ProcessingPage to support batch mode with BatchProcessingPanel
- Update UploadPage to initialize batch state for multi-file uploads
- Add i18n translations for batch processing (zh-TW, en-US)

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

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

openspec/specs/frontend-ui/spec.md

@@ -89,3 +89,100 @@ The application MAY display:
 - **WHEN** any page describes product features
 - **THEN** the descriptions are factual and verifiable
 
+### Requirement: Batch Processing Support
+
+The system SHALL support batch processing of multiple uploaded files with a single configuration.
+
+After uploading multiple files, the user SHALL be able to:
+- Configure processing settings once for all files
+- Start processing all files with one action
+- Monitor progress of all files in a unified view
+
+#### Scenario: Multiple files uploaded
+- **WHEN** user uploads multiple files
+- **AND** navigates to processing page
+- **THEN** the system displays batch processing mode
+- **AND** shows all pending tasks in a list
+
+#### Scenario: Batch configuration
+- **WHEN** user is in batch processing mode
+- **THEN** user can select a processing strategy (auto/OCR/Direct)
+- **AND** user can configure layout model for OCR tasks
+- **AND** user can configure preprocessing for OCR tasks
+- **AND** settings apply to all applicable tasks
+
+---
+
+### Requirement: Batch Processing Strategy
+
+The system SHALL support three batch processing strategies:
+
+1. **Auto Detection** (default): System analyzes each file and selects optimal track
+2. **Force OCR**: All files processed with OCR track
+3. **Force Direct**: All PDF files processed with Direct track
+
+#### Scenario: Auto detection strategy
+- **WHEN** user selects auto detection strategy
+- **THEN** the system analyzes each file before processing
+- **AND** assigns OCR or Direct track based on file characteristics
+
+#### Scenario: Force OCR strategy
+- **WHEN** user selects force OCR strategy
+- **THEN** all files are processed using OCR track
+- **AND** layout model and preprocessing settings are applied
+
+#### Scenario: Force Direct strategy
+- **WHEN** user selects force Direct strategy
+- **AND** file is a PDF
+- **THEN** the file is processed using Direct track
+
+---
+
+### Requirement: Parallel Processing Limits
+
+The system SHALL enforce different parallelism limits based on processing track:
+
+- Direct Track: Maximum 5 concurrent tasks (CPU-based)
+- OCR Track: Maximum 1 concurrent task (GPU VRAM constraint)
+
+Direct and OCR tasks MAY run simultaneously as they use different resources.
+
+#### Scenario: Direct track parallelism
+- **WHEN** batch contains multiple Direct track tasks
+- **THEN** up to 5 tasks process concurrently
+- **AND** remaining tasks wait in queue
+
+#### Scenario: OCR track serialization
+- **WHEN** batch contains multiple OCR track tasks
+- **THEN** only 1 task processes at a time
+- **AND** remaining tasks wait in queue
+
+#### Scenario: Mixed track processing
+- **WHEN** batch contains both Direct and OCR tasks
+- **THEN** Direct tasks run in parallel pool (max 5)
+- **AND** OCR tasks run in serial queue (max 1)
+- **AND** both pools operate simultaneously
+
+---
+
+### Requirement: Batch Progress Display
+
+The system SHALL display unified progress for batch processing.
+
+Progress display SHALL include:
+- Overall progress (completed / total)
+- Count by status (processing, completed, failed)
+- Individual task status list
+- Estimated time remaining (optional)
+
+#### Scenario: Batch progress monitoring
+- **WHEN** batch processing is in progress
+- **THEN** user sees overall completion percentage
+- **AND** user sees count of tasks in each status
+- **AND** user sees status of each individual task
+
+#### Scenario: Batch completion
+- **WHEN** all tasks in batch are completed or failed
+- **THEN** user sees final summary
+- **AND** user can navigate to results page
+