feat: Add comprehensive image support for TUI mode

Problem

Currently, image support in TUI mode is limited and doesn't handle common use cases well:

1. Drag-and-drop doesn't work properly - When dragging image files into the terminal, they either don't get recognized or cause immediate submission
2. Complex filenames fail - Files with spaces, parentheses, or special characters (e.g., Screenshot 2025-01-10 at 12.30.45 PM.png) aren't handled correctly
3. No inline image detection - Users can't simply type image paths in their messages
4. Build issues on stable Rust - The codebase uses unstable let-chains syntax that prevents compilation on stable Rust

Solution

This PR adds comprehensive image support to the TUI with the following improvements:

1. Smart Drag-and-Drop

• When users drag image files into the terminal, they now see placeholders like [image: filename.png] in the compose field
• The message isn't submitted immediately - users can add text around the images
• Supports file:// URLs (common when dragging from Finder on macOS)
• Handles terminal escape sequences for special characters

2. Automatic Image Detection

• Detects image paths typed directly in messages
• Example: typing "Look at screenshot.png" automatically includes the image
• Supports both absolute and relative paths
• Checks if files exist before including them

3. Complex Filename Support

• Properly handles filenames with spaces: My Screenshot.png
• Supports parentheses: Screenshot (1).png
• URL decodes special characters from file:// URLs
• Removes terminal escape sequences (\ → , \( → ()

4. Stable Rust Compatibility

• Fixes unstable let-chains syntax in client.rs, error.rs, and protocol.rs
• Code now compiles on stable Rust without requiring nightly features

Technical Details

The implementation adds a pending_images HashMap to track image placeholders and their paths. When a message is submitted, the system:

1. Checks for [image: ...] placeholders from drag-and-drop
2. Scans the text for image file paths
3. Validates all paths exist
4. Includes them as InputItem::LocalImage in the submission

Testing

• ✅ Drag single/multiple images into terminal
• ✅ Type image paths directly in messages
• ✅ Complex filenames with spaces and special characters
• ✅ Mix text and images in the same message
• ✅ Compiles on stable Rust