fix(ui): sync quick insert highlight with input method - #5176

Summary

Fixes inconsistent highlighting in the Quick Insert menu when switching between mouse and keyboard navigation.

Previously, hovering one item while navigating with the arrow keys could make the hovered item appear selected even though Enter executed a different internally active item.

Closes #5175

Changes

• Switch the menu into keyboard-navigation styling when arrow-key navigation starts.
• Return control to mouse highlighting on actual pointer movement and synchronize the active item with the hovered item.
• Clear the input-mode styling when the menu closes.
• Add an end-to-end regression test covering mouse hover, keyboard navigation, the visible highlight, and the heading inserted by Enter.

Expected behavior

• Mouse movement highlights and activates the item under the pointer.
• Arrow-key navigation keeps the keyboard-active item highlighted, even when the pointer remains over another item.
• Enter executes the item that is visibly highlighted.

Testing

• pnpm --filter @muyajs/core lint (passes with existing warnings)
• pnpm --filter @muyajs/core lint:types
• pnpm exec stylelint packages/muya/src/ui/paragraphQuickInsertMenu/index.css
• pnpm --filter @muyajs/core test (1,438 tests passed)
• pnpm --filter @muyajs/core check-circular
• pnpm --filter muya-e2e exec playwright test tests/ui/slash-menu.spec.ts --project=chromium --workers=1 (8 tests passed)

The full CSS lint command still reports an unrelated, pre-existing no-descending-specificity error in src/assets/styles/blockSyntax.css:945; the changed stylesheet passes targeted Stylelint.