refactor(#292 review): extract NAVBAR_COLLAPSE_BREAKPOINT constant (F2)

The AppShell navbar breakpoint and both burger toggles' hiddenFrom/visibleFrom
must be equal, or the sidebar becomes unreachable on tablet widths (the round-1
regression). A comment guarded that before; now a shared const does. Add
NAVBAR_COLLAPSE_BREAKPOINT='md' to sidebar-atom.ts and reference it from the navbar
breakpoint (global-app-shell) + both toggles (app-header). aside.breakpoint and the
sm brand/search gates are intentionally separate contracts, left untouched.

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

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

@@ -12,6 +12,7 @@ import TopMenu from "@/components/layouts/global/top-menu.tsx";
 import { Link } from "react-router-dom";
 import { useAtom } from "jotai";
 import {
+  NAVBAR_COLLAPSE_BREAKPOINT,
   desktopSidebarAtom,
   mobileSidebarAtom,
 } from "@/components/layouts/global/hooks/atoms/sidebar-atom.ts";
@@ -59,7 +60,7 @@ export function AppHeader() {
               // 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"
+              hiddenFrom={NAVBAR_COLLAPSE_BREAKPOINT}
               size="sm"
             />
           </Tooltip>
@@ -69,7 +70,7 @@ export function AppHeader() {
               aria-label={t("Sidebar toggle")}
               opened={desktopOpened}
               onClick={toggleDesktop}
-              visibleFrom="md"
+              visibleFrom={NAVBAR_COLLAPSE_BREAKPOINT}
               size="sm"
             />
           </Tooltip>

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

@@ -6,6 +6,7 @@ import SettingsSidebar from "@/components/settings/settings-sidebar.tsx";
 import { useAtom } from "jotai";
 import {
   APP_NAVBAR_ID,
+  NAVBAR_COLLAPSE_BREAKPOINT,
   asideStateAtom,
   desktopSidebarAtom,
   mobileSidebarAtom,
@@ -94,7 +95,7 @@ export default function GlobalAppShell({
         // off-screen with no horizontal scroll. Collapsing the navbar to a toggle
         // drawer across the whole tablet band frees the full width for content
         // (the mobile drawer is closed by default, so nothing overlaps on load).
-        breakpoint: "md",
+        breakpoint: NAVBAR_COLLAPSE_BREAKPOINT,
         collapsed: {
           mobile: !mobileOpened,
           desktop: !desktopOpened,

apps/client/src/components/layouts/global/hooks/atoms/sidebar-atom.ts

@@ -7,6 +7,13 @@ import { atom } from "jotai";
 // would create a shell -> chat-window -> shell import cycle).
 export const APP_NAVBAR_ID = "app-shell-navbar";
 
+// Single source of truth for the navbar collapse breakpoint. The AppShell navbar
+// `breakpoint` and BOTH burger toggles' `hiddenFrom`/`visibleFrom` MUST use this
+// exact value: if they drift, the sidebar becomes unreachable on tablet widths
+// (the round-1 regression of #292). Kept here so the shell and the header share
+// one constant the compiler enforces, instead of three hand-synced string literals.
+export const NAVBAR_COLLAPSE_BREAKPOINT = "md";
+
 export const mobileSidebarAtom = atom<boolean>(false);
 
 export const desktopSidebarAtom = atomWithWebStorage<boolean>(