perf(P6B.T8): virtualize message list with react-virtuoso

Switches the ConversationPage chat list from a full O(N) render to
windowed rendering via react-virtuoso. On long histories only the
visible rows (plus a 400px overscan buffer) live in the DOM, ending the
scroll jank and layout thrashing that hit conversations with >500
messages.

Preserved behaviors:
- Newest message visible on open via initialTopMostItemIndex.
- Auto-scroll on send via a pending-count-based effect (the old
  setStickToBottom + useLayoutEffect pattern doesn't apply now that
  Virtuoso owns the scroll element).
- Realtime auto-follow only when scrolled to bottom (followOutput).
- 'New messages while away' counter + 'jump to newest' pill via
  atBottomStateChange.
- Pinned-message / reply / search jumps via virtuosoRef.scrollToIndex;
  expands displayCount on the fly if the target is outside the
  rendered slice. Flash highlight unchanged.
- Load-older infinite scroll via Virtuoso startReached (replaces the
  IntersectionObserver-on-sentinel pattern).
- Per-conversation position memory now keys on row index instead of
  pixel scrollTop (the latter isn't meaningful under virtualization).

Also wires the PinnedMessagesPanel onJump callback (previously a TODO
that just closed the panel) into jumpToMessage, since virtualization
made the smooth-scroll-from-pinned UX easy to deliver as a side
benefit.

pnpm-lock.yaml

@@ -98,6 +98,9 @@ importers:
       react-router-dom:
         specifier: ^6.28.0
         version: 6.30.3(react-dom@18.3.1(react@18.3.1))(react@18.3.1)
+      react-virtuoso:
+        specifier: ^4.18.7
+        version: 4.18.7(react-dom@18.3.1(react@18.3.1))(react@18.3.1)
       zustand:
         specifier: ^5.0.1
         version: 5.0.12(@types/react@18.3.28)(react@18.3.1)(use-sync-external-store@1.6.0(react@18.3.1))
@@ -5420,6 +5423,12 @@ packages:
     peerDependencies:
       react: ^18.3.1
 
+  react-virtuoso@4.18.7:
+    resolution: {integrity: sha512-xNF5zDGEEIMB7cKwcen/pLig0YDf6OnfFrVgKFa7sHPf9fRem0CaLshyObbBcP88jzn0enavL39EgplgdyT21g==}
+    peerDependencies:
+      react: '>=16 || >=17 || >= 18 || >= 19'
+      react-dom: '>=16 || >=17 || >= 18 || >=19'
+
   react@18.3.1:
     resolution: {integrity: sha512-wS+hAgJShR0KhEvPJArfuPVN1+Hz1t0Y6n5jLrGQbkb4urgPE/0Rve+1kMB1v/oWgHgm4WIcV+i7F2pTVj+2iQ==}
     engines: {node: '>=0.10.0'}
@@ -12820,6 +12829,11 @@ snapshots:
       react-shallow-renderer: 16.15.0(react@18.3.1)
       scheduler: 0.23.2
 
+  react-virtuoso@4.18.7(react-dom@18.3.1(react@18.3.1))(react@18.3.1):
+    dependencies:
+      react: 18.3.1
+      react-dom: 18.3.1(react@18.3.1)
+
   react@18.3.1:
     dependencies:
       loose-envify: 1.4.0