feat(windows): add wallpaper reveal control

[CCDawn]

What changed

• add MP4/WebM wallpaper reveal controls to the Windows tray and persist the value with each theme
• keep video media at its original opacity while fading the full-window foreground veil
• update the renderer through CSS variables without retransmitting the full video
• improve native and secondary titlebar contrast over dynamic frames
• extend runtime verification, PowerShell coverage, renderer tests, documentation, and changelog entries

Why

The first slider implementation changed the video element opacity itself. That made 100% hide the wallpaper while the actual white/theme veil remained above it. The control now targets the foreground surfaces instead: at 100% reveal, the video remains at opacity: 1 and the main/sidebar veil resolves to transparent.

User impact

Users can tune how much of a static or dynamic wallpaper is exposed from the Dream Skin tray. At 100%, the wallpaper is shown at its original opacity while interactive controls retain their own readability surfaces.

Validation

• node --check windows/assets/renderer-inject.js
• node --check windows/scripts/injector.mjs
• node windows/tests/renderer-inject.test.mjs
• node windows/tests/dynamic-media.test.mjs
• powershell.exe -NoProfile -ExecutionPolicy Bypass -File windows/tests/run-tests.ps1
• live CDP verification: wallpaper reveal 1, video opacity 1, transparent main/sidebar gradients, no document overflow
• hot deployment preserved all running Codex process IDs