Polish authoring previews and release updates

[S1mplector]

What changed

• adds a responsive Puppeteer live-preview readout with timing, selection, expression, and upcoming-key information
• improves runtime startup loading and runtime-error recovery presentation and keyboard controls
• adds a version-aware, themed What's New dialog shared by Launcher and Editor, with persisted one-time display behavior
• removes the obsolete Engine Hub announcements screen, bell badge, read-state storage, parser, bundled announcement file, and supporting icon code
• updates focused tests and Engine Hub documentation

Why

Puppeteer authoring needed more immediate timeline feedback, runtime startup and errors needed clearer recovery-oriented presentation, and update communication is better tied to concrete application versions than a separate Hub notification inbox.

User impact

Authors get richer live context while editing, calmer startup feedback, more actionable runtime errors, and a concise release summary once per version. The Engine Hub header and Help menu are simpler without the redundant announcements feature.

Validation

• ./gradlew :core:test
• ./gradlew :fx:test
• ./gradlew :hub:test
• focused editor tests for Puppeteer readouts, What's New catalog/version behavior, preferences persistence, startup paths, Aero icons, and dual-theme CSS
• git diff --check