This commit changes `WatchSource` signatures into `Readonly<Ref>`s.
It provides two important benefits:
1. Eliminates the possibility of `undefined` states, that's result of
using `WatchSource`s. This previously required additional null checks.
By using `Readonly<Ref>`, the state handling becomes simpler and less
susceptible to null errors.
2. Optimizes performance by using references:
- Avoids the reactive layer of `computed` references when not needed.
- The `watch` syntax, such as `watch(() => ref.value)`, can introduce
side effects. For example, it does not account for `triggerRef` in
scenarios where the value remains unchanged, preventing the watcher
from running (vuejs/core#9579).
This commit fixes an issue where the check state of categories was lost
when toggling between card and tree views. This is solved by immediately
emitting node state changes for all nodes. This ensures consistent view
transitions without any loss of node state information.
Furthermore, this commit includes added unit tests for the modified code
sections.
