Add drag & drop image support with multimodal vision capabilities

[BoQsc]

Implements comprehensive image upload functionality matching Claude Code CLI behavior:

• Extended shared types with ImageData and MultimodalMessage interfaces for type safety
• Created image validation utilities with support for JPEG, PNG, GIF, WebP formats
• Enhanced ChatInput component with drag & drop interface and image previews
• Updated backend chat handler to process multimodal messages via Claude SDK
• Added image display support in MessageComponents with grid layout
• Integrated multimodal state management in ChatPage for seamless UX

Users can now drag images directly into the chat or click to browse files, with Claude analyzing them through its vision capabilities just like the original CLI tool.

🤖 Generated with Claude Code

Description

Brief description of the changes in this PR.

Type of Change

Please add the appropriate label(s) to this PR and check the relevant box(es):

• 🐛 bug - Bug fix (non-breaking change which fixes an issue)
• ✨ feature - New feature (non-breaking change which adds functionality)
• 💥 breaking - Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 documentation - Documentation update
• ⚡ performance - Performance improvement
• 🔨 refactor - Code refactoring
• 🧪 test - Adding or updating tests
• 🔧 chore - Maintenance, dependencies, tooling

Changes Made

• List key changes
• Include any breaking changes
• Mention new dependencies or configuration changes

Testing

• Tests pass locally (make test)
• Code is formatted (make format)
• Code is linted (make lint)
• Type checking passes (make typecheck)
• All quality checks pass (make check)
• Manual testing performed (describe what was tested)

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated tests for my changes
• All tests pass

Screenshots (if applicable)

Add screenshots to help explain your changes.

Additional Notes

Any additional information, dependencies, or context needed for reviewers.