Add Emoji Highlighter

[maxmmitchell]

• Add logic to make these three new stock brushes available in Cahier stock brush store, leveraging the existing emoji highlighter brush from the Ink API
• Add requisite textures for three emojis: heart, star, and the ever-popular poop emoji
• Add emoji highlighter as an option when drawing in a Cahier note, and in the brush designer canvas
• Add submenu to select from three available options
• Add SVGs to adorn the new menu options

Similar menu options will be added to brush graph within that branch after this merges and I can pull the changes in.

[gemini-code-assist]

Code Review

This pull request introduces Emoji Highlighter brushes (Heart, Star, and Poop) by adding new vector assets, texture mappings, and serialization support. It also implements submenus in the Brush Designer and Drawing Toolbox to select these brushes. Feedback highlights a potential serialization issue when using dynamic brush instances as map keys, efficiency concerns and incorrect color literals in the ViewModel, visual inconsistency in the Poop emoji icon, and the use of brittle hardcoded UI offsets.