fix(sidebar): keep navigation hover under pointer

[kilockok]

What changed

• Track the pointer target explicitly for settings navigation items while the sidebar scrolls.
• Paint sidebar navigation hover from the explicit target instead of Chromium's delayed :hover state.
• Keep press scaling while making the navigation hover background switch immediately.

Why

Chromium can retain the previous CSS :hover target during compositor scrolling. The shared 150ms ghost-button background transition also made rapid pointer movement appear to trail behind the cursor.

Validation

• pnpm exec eslint src/lib/scroll-hover.ts src/components/sidebar/SidebarContent.vue src/components/settings/NavItem.vue
• pnpm type-check
• git diff --check

⚠️ No human QA — this PR has not been verified by a human yet. Remove this line once a human confirms the happy path.