refactor(team): wire scroll owners for MessagesPanel layouts

First step toward virtualizing ActivityTimeline. Makes the real scroll
container observable per layout, without changing behavior.

- `TeamDetailView` forwards `contentRef` to `MessagesPanel` as
  `inlineScrollContainerRef`. `MessagesPanel` accepts it as an optional
  prop (unused in this release) so a follow-up can wire the inline
  viewport to virtualization consumers.
- `MessagesPanel` creates `bottomSheetScrollRef` and passes it to
  `Sheet.Content scrollRef`. react-modal-sheet merges it with its
  internal scroll ref, so the element stays the same DOM node; this
  only exposes it to us.
- Sidebar already owns `sidebarScrollRef`; no change there.

Behavior is unchanged — this only exposes refs for the follow-up that
will thread a viewport contract into ActivityTimeline.

src/renderer/components/team/TeamDetailView.tsx

@@ -39,8 +39,8 @@ import {
 } from '@renderer/store/slices/teamSlice';
 import { createChipFromSelection } from '@renderer/utils/chipUtils';
 import { sumContextInjectionTokens } from '@renderer/utils/contextMath';
-import { formatProjectPath } from '@renderer/utils/pathDisplay';
 import { buildMemberColorMap } from '@renderer/utils/memberHelpers';
+import { formatProjectPath } from '@renderer/utils/pathDisplay';
 import { buildTaskCountsByOwner, normalizePath } from '@renderer/utils/pathNormalize';
 import { nameColorSet } from '@renderer/utils/projectColor';
 import { resolveProjectIdByPath } from '@renderer/utils/projectLookup';
@@ -2011,6 +2011,7 @@ export const TeamDetailView = ({
       onReplyToMessage: handleReplyToMessage,
       onRestartTeam: handleRestartTeam,
       onTaskIdClick: handleTaskIdClick,
+      inlineScrollContainerRef: contentRef,
     }),
     [
       activeMembers,

src/renderer/components/team/messages/MessagesPanel.tsx

@@ -1,4 +1,13 @@
-import { memo, useCallback, useEffect, useLayoutEffect, useMemo, useRef, useState } from 'react';
+import {
+  memo,
+  type RefObject,
+  useCallback,
+  useEffect,
+  useLayoutEffect,
+  useMemo,
+  useRef,
+  useState,
+} from 'react';
 import { Sheet, type SheetRef } from 'react-modal-sheet';
 
 import { Badge } from '@renderer/components/ui/badge';
@@ -91,6 +100,13 @@ interface MessagesPanelProps {
   onRestartTeam?: () => void;
   /** Callback when a task ID link is clicked. */
   onTaskIdClick?: (taskId: string) => void;
+  /**
+   * Scroll container owned by the parent view when `position === 'inline'`.
+   * MessagesPanel does not own this element — the viewport lives in
+   * TeamDetailView's content scroll area. Plumbed for future viewport
+   * consumers (virtualization); unused in this release.
+   */
+  inlineScrollContainerRef?: RefObject<HTMLDivElement | null>;
 }
 
 export function reconcilePendingRepliesByMember(
@@ -214,6 +230,11 @@ export const MessagesPanel = memo(function MessagesPanel({
   const sidebarScrollRef = useRef<HTMLDivElement | null>(null);
   const bottomSheetRef = useRef<SheetRef>(null);
   const bottomSheetStickyTopRef = useRef<HTMLDivElement | null>(null);
+  // Scroll container inside `Sheet.Content` for the bottom-sheet layout.
+  // react-modal-sheet merges this ref with its own internal scroll ref.
+  // Held here so future viewport consumers (virtualization) can observe the
+  // true scrolling element in bottom-sheet mode.
+  const bottomSheetScrollRef = useRef<HTMLDivElement | null>(null);
   const handleExpandContent = useCallback(() => {
     // no-op: user is reading expanded content, not composing
   }, []);
@@ -1063,6 +1084,7 @@ export const MessagesPanel = memo(function MessagesPanel({
             <Sheet.Content
               className="min-h-0 bg-[var(--color-surface-sidebar)]"
               scrollClassName="flex min-h-full flex-col"
+              scrollRef={bottomSheetScrollRef}
               disableDrag={(state) => state.scrollPosition !== 'top'}
             >
               <div