feat: Add drag-and-drop CSV upload for batch stream creation

[Austinaminu2]

Description

The batch stream creation feature currently requires clicking a file input to select a CSV. Adding drag-and-drop support would improve the UX significantly, especially for users creating many streams at once (payroll, airdrops).

Proposed Solution

• Add a drop zone component to the batch creation page
• Support drag-and-drop for .csv files
• Show visual feedback (border highlight, icon change) when a file is dragged over
• Validate file type on drop (reject non-CSV with a clear error)
• Show a preview table of parsed rows before submission
• Keep the existing click-to-upload as a fallback

Design Reference

┌─────────────────────────────────────┐
│                                     │
│    📄 Drag & drop your CSV here    │
│       or click to browse            │
│                                     │
│    Supports: .csv (max 500 rows)    │
└─────────────────────────────────────┘

Acceptance Criteria

• Drop zone accepts .csv files via drag-and-drop
• Visual feedback on drag-over (highlighted border, background change)
• File type validation with clear error for non-CSV files
• Preview table shows parsed data before submission
• Click-to-browse fallback remains functional
• Works on mobile (fallback to file input only)