fix: anchor quit sheet slide animation

[Wangnov]

Summary

• Wrap Sheet content in a bottom-anchored frame so the quit confirmation remains a real bottom sheet slide-up.
• Use explicit sheet enter/exit keyframes and a bottom skirt to cover compositor gaps during the slide.
• Align the global quit scrim with the window gutter variables.

Validation

• npm run check
• npm test -- Sheet
• npm run tauri:build
• codex review --base main: no findings

Note: codex review was run in a clean temporary worktree; its attempt to run vitest there failed because that worktree did not have node_modules, but the test passed in the main workspace.