feat(mobile): right-rail as drawer on mobile, header toggle button

Reverts v1.6.1's max-md:hidden wrapper around RightRail. On mobile,
RightRail now renders as a fixed right-side drawer (w-[85vw],
max-w-sm) toggled by a new FolderTree button in the Session header.

- New useRightRailDrawer hook mirrors useSidebarDrawer (Context +
  auto-close on route change).
- New MobileRightRailBackdrop component in App.tsx mirrors the
  existing MobileBackdrop for the left sidebar.
- RightRail computes an isOpen synthesis: on mobile, reads the
  drawer Context; on desktop, reads the persistent internal state.
  The existing tree-load effect and open_file_in_browser
  subscription share this plumbing via openRail / closeRail
  helpers.
- The desktop floating chevron handle is hidden on mobile (the
  Session header's FolderTree button replaces it).
- Session header gains a mobile-only FolderTree button after the
  ModelPicker, calling toggle() on the drawer Context.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/pages/Session.tsx

@@ -1,11 +1,12 @@
 import { useEffect, useState } from 'react';
 import { Link, useNavigate, useParams } from 'react-router-dom';
-import { ChevronRight, Menu } from 'lucide-react';
+import { ChevronRight, FolderTree, Menu } from 'lucide-react';
 import { api } from '@/api/client';
 import type { Project, Session as SessionType } from '@/api/types';
 import { sessionEvents } from '@/hooks/sessionEvents';
 import { useActivePane } from '@/hooks/useActivePane';
 import { useSidebarDrawer } from '@/hooks/useSidebarDrawer';
+import { useRightRailDrawer } from '@/hooks/useRightRailDrawer';
 import { useViewport } from '@/hooks/useViewport';
 import { Workspace } from '@/components/Workspace';
 import { ModelPicker } from '@/components/ModelPicker';
@@ -19,6 +20,7 @@ export function Session() {
   const [editingName, setEditingName] = useState(false);
   const active = useActivePane();
   const { setOpen: setDrawerOpen } = useSidebarDrawer();
+  const { toggle: toggleRightRail } = useRightRailDrawer();
   const { isMobile } = useViewport();
 
   useEffect(() => {
@@ -176,6 +178,18 @@ export function Session() {
             </div>
           )}
         </div>
+
+        {/* File browser toggle — mobile only */}
+        {isMobile && (
+          <button
+            type="button"
+            onClick={toggleRightRail}
+            className="inline-flex items-center justify-center -mr-1 min-w-[44px] min-h-[44px] rounded text-muted-foreground hover:bg-muted hover:text-foreground shrink-0"
+            aria-label="Toggle file browser"
+          >
+            <FolderTree className="size-5" />
+          </button>
+        )}
       </header>
 
       {id && session && (