fix(coach): bound the conversation scroll region so the composer stays pinned

The message thread used h-full inside a plain-block wrapper, so its height
resolved against an auto-height parent: the scroll container grew with its
content instead of scrolling, pushing the composer down and leaving a stray
scrollbar on the page.

Make the thread wrapper a flex column and switch the thread (and its empty
state) from h-full to min-h-0 flex-1, so the scroll region takes a bounded
track from the flex parent. The conversation now scrolls cleanly inside the
card while the composer stays pinned to the bottom.

src/components/insights/coach-panel/coach-drawer-body.tsx

@@ -146,7 +146,14 @@ export function CoachDrawerBody({
           </Button>
         </div>
 
-        <div className="min-h-0 flex-1">{thread}</div>
+        {/* v1.18.6.1 — the thread wrapper must be a flex column itself so the
+            `<MessageThread>` scroll region resolves a bounded height. The
+            wrapper used to be a plain block, so the thread's `h-full` resolved
+            against an auto-height parent and the scroll container grew with its
+            content instead of scrolling — pushing the composer off-screen and
+            leaving a stray scrollbar on the page. `flex min-h-0 flex-1 flex-col`
+            gives the thread a definite track to scroll within. */}
+        <div className="flex min-h-0 flex-1 flex-col">{thread}</div>
         {/* Composer pinned to the bottom. */}
         <div
           data-slot="coach-drawer-composer"

src/components/insights/coach-panel/message-thread.tsx

@@ -342,7 +342,7 @@ export function MessageThread({
         data-slot="coach-message-thread"
         role="status"
         aria-live="polite"
-        className="text-muted-foreground flex h-full flex-col items-center justify-center gap-3 px-6 text-center"
+        className="text-muted-foreground flex min-h-0 flex-1 flex-col items-center justify-center gap-3 px-6 text-center"
       >
         <div
           aria-hidden="true"
@@ -362,7 +362,10 @@ export function MessageThread({
       ref={scrollerRef}
       data-slot="coach-message-thread"
       className={cn(
-        "flex h-full flex-col gap-4 overflow-y-auto px-4 py-4 sm:px-6",
+        // v1.18.6.1 — `min-h-0 flex-1` (not `h-full`) so the scroll region
+        // resolves its height from the flex parent rather than a 100%-of-auto
+        // chain that let the thread grow instead of scroll.
+        "flex min-h-0 flex-1 flex-col gap-4 overflow-y-auto px-4 py-4 sm:px-6",
         // v1.18.6 (CCH-01) — on the now full-width page surface a thread
         // stretched edge-to-edge sprawled the prose to an unreadable
         // measure. Centre the content on a comfortable max width (Claude-