Streamline UI: remove welcome screen, brush types, and layer reordering buttons

[Microck]

Description

This PR streamlines the UI based on user feedback to reduce friction and help users jump straight into animating. Removes the welcome screen, simplifies brush options, and removes layer reordering buttons while keeping the 5-step tutorial for new users.

Type of Change

• Code refactoring
• Performance improvement
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Changes Made

• Welcome Screen: Removed home modal on startup; app now auto-creates a 16:9 project so users can start animating immediately
• Brush Types: Removed brush shape selector (circle, square, diamond, etc.) - brush now defaults to circle only for simpler UX
• Layer Up/Down Buttons: Removed the ▲/▼ layer reordering buttons; layer hierarchy is now fixed. Users can still reorder color swatches within layers via drag (works on mobile via pointer events)
• Lasso Performance: Optimized lasso tool for mobile by removing expensive dashed line rendering and adding requestAnimationFrame throttling to batch preview redraws

Test Configuration:

• OS: Linux
• Browser: Chrome

Screenshots (if applicable)

N/A - UI simplifications

Checklist

• My code follows the style guidelines of this project
• 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
• My changes generate no new warnings
• I have tested my changes thoroughly
• Any dependent changes have been merged and published