feat: add custom PNG/JPEG/ICNS icon support

[kevintraver]

Summary

Add support for custom image files (PNG, JPEG, ICNS) as a third icon type alongside existing app icons and SF Symbols.

Motivation

Sometimes you want to use a shell command or script to open a website or app, but still display a specific icon rather than the generic terminal icon. For example:

• A command that opens Hacker News in your browser — you'd want to show the HN favicon
• A script that launches a specific app with custom flags — you'd want to show that app's icon
• A URL scheme action where the favicon doesn't load correctly

Custom icons solve this by letting you specify any PNG, JPEG, or ICNS file as the icon.

Changes

• Add CustomIconRenderer for rendering custom icons with proper sizing and rounded corners
• Add CustomIconImage SwiftUI view for displaying custom icons in the cheatsheet
• Add "Custom Icon..." option to the icon picker menu
• Add file picker handlers for both Actions and Groups
• Add validation to ensure selected files are valid images (shows error if not)
• Icons are rendered at the correct size with macOS-style rounded corners to match app bundle icons

How to use

1. Click the icon button on any action or group
2. Select "Custom Icon..."
3. Choose a PNG, JPEG, or ICNS file
4. The custom icon will display in both the settings editor and cheatsheet

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