Feat: Batch Operations for Images

### Is there an existing issue for this?
- [x] I have searched the existing issues

### Description
Add ability to select multiple images and perform bulk actions (delete, tag, move to album).

### Problem
Users can only perform actions on one image at a time, which is inefficient for large collections.

### Solution
- Selection mode toggle in navbar
- Checkboxes on images for multi-select
- Floating toolbar with batch actions: Delete, Tag, Move to Album
- Keyboard shortcuts: Ctrl+A (select all), Escape (deselect)

### Technical Details
**Frontend:**
- Redux state for selection
- BatchToolbar component
- BatchTagDialog component
- Update ImageCard with checkbox

**Backend:**
- DELETE /images/batch-delete
- POST /images/batch-tag
- POST /images/batch-move

### Record

- [x] I agree to follow this project's Code of Conduct

### Checklist before Submitting

- [x] Have you updated docs for it?
- [ ] Have you added unit tests?
- [ ] Have you made sure unit tests pass?
- [x] Have you made sure code formatting is correct?
- [ ] Do Your changes passes all tests?