fix(coach): give the chat page more width and fix the rail scrollbar

The Coach page read as cut off against other pages and the left
conversation scrollbar looked broken.

Width: the page sat inside the shared max-w-screen-xl content frame with
a tall pb-20 FAB gutter that does nothing on /coach (the FAB hides
there). Break out of the frame so the chat claims the full content width
and viewport height, edge-to-edge on small screens and a contained card
from sm up, and centre the thread on a comfortable reading measure.

Scrollbar: the page-surface history rail was wrapped in an overflow-auto
container while the rail itself is a flex column with its own
overflow-auto list — two nested scroll regions. The inner list's height
never resolved, so its scrollbar rendered against an indeterminate track
and the search field scrolled away. Drop the outer scroll: the rail
keeps a fixed search header and one internal list-scroll.

src/app/coach/page.tsx

@@ -63,15 +63,35 @@ export default function CoachPage() {
     <div
       data-slot="coach-page"
       data-tour-id="coach-hero"
-      // v1.18.1 (W-COACH-UI C1/C3) — the card was capped at
-      // `100dvh-13rem`, leaving a band of wasted space below it on a
-      // standalone route that no longer carries the Insights tab strip.
-      // Claim the page height down to a comfortable bottom gutter so the
-      // thread + rail fill the viewport, and lift the card with a soft
-      // shadow so it reads as one contained surface rather than a thin
-      // outline.
-      className="bg-card flex h-[calc(100dvh-8rem)] min-h-[32rem] flex-col overflow-hidden rounded-2xl border shadow-sm"
+      // v1.18.6 (CCH-01) — Marc: the Coach read "cut off" vs other pages
+      // and wanted more width / a focused Claude-like surface. The page
+      // sits inside `<AuthShell>`'s shared `mx-auto max-w-screen-xl px-4
+      // pt-6 pb-20 md:px-6` content frame, which boxed the chat into the
+      // same narrow column every dashboard card uses and left a tall dead
+      // band below the card (the `pb-20` FAB gutter, pointless here since
+      // the FAB hides on `/coach`). Break out of that frame: negative
+      // margins cancel the container's horizontal + top/bottom padding so
+      // the chat claims the full content width and the viewport height,
+      // then a light inner padding keeps it off the window edge. The card
+      // itself drops its rounded border on small screens (edge-to-edge,
+      // app-like) and keeps the contained card look from `sm+`.
+      //
+      // v1.18.1 (W-COACH-UI C1/C3) — the card formerly capped at
+      // `100dvh-13rem`, leaving wasted space below it; we now fill the
+      // reclaimed height.
+      className={
+        // Cancel the AuthShell container's padding (`-mt-6` top,
+        // `-mb-20` bottom, `-mx-4 md:-mx-6` sides) so the surface is
+        // full-bleed, then re-pad lightly. The height is the full
+        // viewport minus the top bar (`4rem`) and the small gutters.
+        "-mx-4 -mt-6 -mb-20 flex h-[calc(100dvh-4rem)] min-h-[32rem] flex-col md:-mx-6 " +
+        "px-2 pt-2 pb-3 sm:px-4 sm:pt-4 sm:pb-4"
+      }
     >
+      <div
+        data-slot="coach-page-card"
+        className="bg-card flex min-h-0 flex-1 flex-col overflow-hidden rounded-none border-y shadow-sm sm:rounded-2xl sm:border"
+      >
       <CoachConversation
         surface="page"
         autoFocusComposer
@@ -95,7 +115,8 @@ export default function CoachPage() {
             </Button>
           </>
         }
-      />
+        />
+      </div>
     </div>
   );
 }

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

@@ -78,7 +78,18 @@ export function CoachDrawerBody({
           <h2 className="border-border/70 text-muted-foreground flex h-14 shrink-0 items-center border-b px-4 text-xs font-medium tracking-wide uppercase">
             {t("insights.coach.historyTitle")}
           </h2>
-          <div className="min-h-0 flex-1 overflow-y-auto">{historyRail}</div>
+          {/* v1.18.6 (CCH-01) — the wrapping `overflow-y-auto` div here
+              was the broken left scrollbar: `<HistoryRail>` is itself a
+              `flex h-full flex-col` with its OWN `overflow-y-auto` list,
+              so this nested it inside a second scroll container. The
+              outer container scrolled the whole rail (search field
+              included) against an `h-full` child whose height resolved to
+              its content, leaving the inner list's `flex-1 min-h-0`
+              without a bounded track — the scrollbar rendered against an
+              indeterminate height and read as half-broken. Drop the outer
+              scroll: the rail keeps a fixed search header and its single
+              internal list-scroll is the only scroll region. */}
+          <div className="flex min-h-0 flex-1 flex-col">{historyRail}</div>
         </aside>
       )}
 

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

@@ -363,6 +363,13 @@ export function MessageThread({
       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 (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-
+        // like) via an `mx-auto` inner gutter; the scrollbar still rides
+        // the surface edge. The drawer surface is already narrow, so the
+        // cap only bites on the wide page surface.
+        "[&>*]:mx-auto [&>*]:w-full [&>*]:max-w-3xl",
         "scroll-smooth",
       )}
     >