refactor: migrate What's New to control-strip and extract viewport SCSS

[willeastcott]

Summary

• Migrates the "What's New" popup from a custom Container+Label+Button implementation to a proper PCUI Button inside a control-strip Container, matching the existing viewport toolbar pattern
• Uses a real <span> dismiss icon inside the button DOM (since pseudo-elements cannot receive their own click events), with stopPropagation to separate the dismiss click from the button click
• Extracts all viewport overlay styles (.viewport, .control-strip, .whoisonline, .chat-widget) from _editor-main.scss into a new _editor-viewport.scss partial
• Deletes _editor-release-notes.scss (absorbed into _editor-viewport.scss)

Test plan

• Verify the "What's New" button appears in the bottom-right of the viewport on first load (or when localStorage key is cleared)
• Click the main button area -- should open GitHub release notes URL and dismiss the button
• Click the X dismiss icon -- should dismiss the button without opening the URL
• Verify control-strip buttons in the top-left (scene tools) and top-right (render/camera/launch) are unchanged
• Verify chat widget and who-is-online overlays render correctly
• Verify the button has the orange accent color with uppercase text

[Copilot]

Pull request overview

This PR refactors the "What's New" notification to use the established control-strip pattern and extracts viewport-related SCSS into a dedicated file for better organization.

Changes:

• Migrated "What's New" popup from a custom picker implementation to a proper PCUI Button within a control-strip Container, matching existing viewport toolbar patterns
• Extracted all viewport overlay styles (viewport, control-strip, whoisonline, chat-widget) from _editor-main.scss into a new dedicated _editor-viewport.scss file
• Replaced the GitHub icon reference from E191 (bake icon) to E259 (proper GitHub icon) for better semantic meaning

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/editor/toolbar/toolbar-whats-new.ts	New implementation using control-strip pattern with proper PCUI Button and custom dismiss icon DOM element
src/editor/pickers/picker-release-notes.ts	Removed old custom picker implementation
src/editor/index.ts	Moved import from pickers section to toolbar section
sass/editor/_editor-viewport.scss	New file consolidating all viewport-related styles including new bottom-right control-strip positioning and whats-new button styles
sass/editor/_editor-release-notes.scss	Removed old standalone styles file
sass/editor/_editor-main.scss	Removed viewport overlay styles (moved to _editor-viewport.scss)
sass/editor.scss	Updated import to reference new viewport SCSS file

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.