feat: complete Dream Skin companion and dock backgrounds

[ShaohengXui]

Supersedes #1613 because GitHub did not synchronize that PR after follow-up commits were pushed to its fork branch.

Changes

• configure an optional custom image beside the current composer
• support the current Codex composer DOM
• keep Dream Skin active when the left sidebar is hidden
• extend the wallpaper treatment to the right dock and bottom terminal panel
• preserve readable terminal, composer, message, dialog, and menu surfaces
• reconcile and clean dock marker classes as panels open, close, or resize

Verification

• manager tests: 35 passed, 0 failed
• TypeScript check: passed
• production Vite build: passed
• renderer syntax checks: passed
• Rust formatting: passed
• live CDP: right markers 3, bottom markers 3, all structural layers transparent
• terminal background: translucent alpha 0.58
• visual screenshot verified with both docks open

Windows Rust execution is delegated to GitHub Actions because the local machine does not have MSVC link.exe.

[ShaohengXui]

@BigPizzaV3 ?? Dream Skin ????????,Windows?macOS x64?macOS arm64 CI ????,PR ??????????????:????????????????????????????????????????????????????????????,????,???