fix(ui): match sidebar toggle breakpoint to navbar (md) so the tablet drawer opens

Follow-up to the navbar sm->md change on this branch: the two header sidebar
toggles were still gated at sm, so in the 768-991 band the DESKTOP toggle was
shown while the navbar used the MOBILE drawer collapse state — clicking it
flipped the wrong atom and the drawer could not be opened (sidebar unreachable
at 768/820, caught by QA). Gate the mobile toggle hiddenFrom=md and the desktop
toggle visibleFrom=md so the mobile toggle drives the drawer across the whole
tablet band.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/components/layouts/global/app-header.tsx

@@ -53,7 +53,13 @@ export function AppHeader() {
               aria-label={t("Sidebar toggle")}
               opened={mobileOpened}
               onClick={toggleMobile}
-              hiddenFrom="sm"
+              // Must match the AppShell navbar breakpoint (md). The navbar
+              // collapses to the MOBILE drawer below md, so the mobile toggle
+              // (which flips mobileOpened) must be the one visible across the
+              // whole <md band — otherwise at 768-991 the desktop toggle showed
+              // but flipped the wrong atom, leaving the drawer unopenable (the
+              // regression from the initial sm->md navbar change).
+              hiddenFrom="md"
               size="sm"
             />
           </Tooltip>
@@ -63,7 +69,7 @@ export function AppHeader() {
               aria-label={t("Sidebar toggle")}
               opened={desktopOpened}
               onClick={toggleDesktop}
-              visibleFrom="sm"
+              visibleFrom="md"
               size="sm"
             />
           </Tooltip>