[codex] add 3D skin library and safer portable package

[ExtremestoneGG]

What changed

• adds a dedicated Skins page with an interactive 3D viewer powered by skinview3d
• stores multiple offline skins locally and allows one-click preview, activation, and deletion
• migrates the currently selected offline skin into the reusable library
• adds a recommended portable ZIP containing the launcher, WebView2Loader.dll, and a short security guide
• keeps the single-file self-extracting portable as an optional convenience build
• publishes SHA-256 checksums and documents Windows SmartScreen behavior honestly
• bumps VEX to 0.9.0 and updates release documentation

Why

Players need an easy way to keep and switch offline skins, while the existing single-file portable can attract stricter browser and antivirus heuristics because it extracts an embedded executable and DLL at runtime. The ZIP distribution is simpler to inspect and is now the recommended portable download.

Validation

• npm run build
• cargo check --manifest-path src-tauri/Cargo.toml
• responsive visual checks at 1280x720 and 800x600
• smoke-tested the portable ZIP and single-file portable on Windows
• scanned installer, ZIP, and single-file portable with Microsoft Defender: no relevant detections
• verified generated SHA-256 checksums and embedded version 0.9.0