Fix WASD and Q/E navigation after focusing the 3D viewport

[JamieW105]

Clicking the 3D viewport previously gave it keyboard focus and then disabled WASD and Q/E: egui reports keyboard capture for any focused widget, including the viewport itself. Allow the focused viewport to receive its own keys, retain hover navigation when no other widget has focus, and pause keyboard navigation when the editor window loses focus. Use the same focus rule for embedded Play input.

Add headless egui regression tests for clicked/hovered viewports, text-field focus and window focus loss. Document controls and recovery steps in the engine guide and the companion website PR: Rustic-Game-Engine/docs#39.

Validation:

• All 32 rustic-editor tests passed, including the three focus regression tests.
• cargo fmt --all -- --check and git diff --check passed.
• cargo clippy --locked -p rustic-editor --all-targets -- -D warnings passed.
• Companion website lint, production build, TypeScript, audit and docs/link checks passed.
• All required engine CI checks passed on Linux and Windows, including rendered viewport verification and application smoke tests.
• Windows CI built a fresh installer, tested toolchain failures and packaged Setup, and uploaded the 21.7 MB windows-installer artifact: https://github.com/Rustic-Game-Engine/engine/actions/runs/38491493926/artifacts/11862971142.

Native desktop interaction has not been manually tested in this workspace.