webclient: replace vue-virtual-scroller with plain content-visibility rendering ...

The virtual scroller's measurement states duplicated item views (parked
copies at translateY(-9999) alongside working ones), which made tool/plan/
thinking cards unclickable and the chat flicker. Both chat messages and
the session sidebar are now plain v-for lists; off-screen rows skip
render/paint via content-visibility: auto, keeping long sessions cheap
without the scroller engine. Library removed from dependencies.