feat(desktop): auto-collapse both sidebars below 600px into hover-reveal

Add a Pane `forceCollapsed` prop — collapses the track without writing to
the store (so the saved open state restores when the window widens) while
keeping hoverReveal alive (unlike `disabled`, which suppresses it).

desktop-controller watches (max-width: 600px) and force-collapses the chat
sidebar + file browser, so on a narrow window both rails get out of the way
and the hover-reveal overlay becomes the way in.

apps/desktop/src/app/desktop-controller.tsx

@@ -8,6 +8,7 @@ import { DesktopInstallOverlay } from '@/components/desktop-install-overlay'
 import { DesktopOnboardingOverlay } from '@/components/desktop-onboarding-overlay'
 import { GatewayConnectingOverlay } from '@/components/gateway-connecting-overlay'
 import { Pane, PaneMain } from '@/components/pane-shell'
+import { useMediaQuery } from '@/hooks/use-media-query'
 import { useSkinCommand } from '@/themes/use-skin-command'
 
 import { formatRefValue } from '../components/assistant-ui/directive-text'
@@ -166,6 +167,10 @@ export function DesktopController() {
   const terminalTakeover = useStore($terminalTakeover)
   const panesFlipped = useStore($panesFlipped)
   const profileScope = useStore($profileScope)
+  // Below 600px there's no room for a docked rail — collapse both sidebars
+  // (without touching their stored open state) so the hover-reveal overlay
+  // becomes the way in. Restores to the saved layout once it's wide again.
+  const narrowViewport = useMediaQuery('(max-width: 600px)')
 
   const routedSessionId = routeSessionId(location.pathname)
   const routeToken = `${location.pathname}:${location.search}:${location.hash}`
@@ -848,6 +853,7 @@ export function DesktopController() {
     <Pane
       defaultOpen={false}
       disabled={!chatOpen}
+      forceCollapsed={narrowViewport}
       hoverReveal
       id="file-browser"
       key="file-browser"
@@ -876,6 +882,7 @@ export function DesktopController() {
     >
       <Pane
         disabled={terminalTakeoverActive}
+        forceCollapsed={narrowViewport}
         hoverReveal
         id="chat-sidebar"
         maxWidth={SIDEBAR_MAX_WIDTH}

apps/desktop/src/components/pane-shell/pane-shell.tsx

@@ -32,6 +32,8 @@ export interface PaneProps {
   defaultOpen?: boolean
   /** Forces the pane closed (track→0, aria-hidden) without writing to the store — for transient route gates. */
   disabled?: boolean
+  /** Like disabled, but keeps hoverReveal alive — collapses the track without writing to the store (e.g. narrow window). */
+  forceCollapsed?: boolean
   /** When collapsed, float the contents over the main column on hover/focus instead of hiding them (track stays 0px). */
   hoverReveal?: boolean
   /** Called with the reveal state whenever a collapsed hoverReveal pane floats in/out. */
@@ -58,6 +60,7 @@ export interface PaneShellProps {
 interface CollectedPane {
   defaultOpen: boolean
   disabled: boolean
+  forceCollapsed: boolean
   id: string
   resizable: boolean
   side: PaneSide
@@ -115,6 +118,7 @@ function collectPanes(children: ReactNode) {
     const entry: CollectedPane = {
       defaultOpen: props.defaultOpen ?? true,
       disabled: props.disabled ?? false,
+      forceCollapsed: props.forceCollapsed ?? false,
       id: props.id,
       resizable: props.resizable ?? false,
       side: props.side,
@@ -129,7 +133,7 @@ function collectPanes(children: ReactNode) {
 
 function trackForPane(pane: CollectedPane, states: Record<string, { open: boolean; widthOverride?: number }>) {
   const stateOpen = states[pane.id]?.open ?? pane.defaultOpen
-  const open = !pane.disabled && stateOpen
+  const open = !pane.disabled && !pane.forceCollapsed && stateOpen
 
   if (!open) {
     return { open: false, track: '0px' }