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/hooks/useRightRailDrawer.tsx

@@ -0,0 +1,35 @@
+import { createContext, useCallback, useContext, useEffect, useState } from 'react';
+import type { ReactNode } from 'react';
+import { useLocation } from 'react-router-dom';
+
+interface RightRailDrawerState {
+  open: boolean;
+  setOpen: (open: boolean) => void;
+  toggle: () => void;
+}
+
+const Ctx = createContext<RightRailDrawerState | null>(null);
+
+export function RightRailDrawerProvider({ children }: { children: ReactNode }) {
+  const [open, setOpen] = useState(false);
+  const location = useLocation();
+
+  // Auto-close on route change. Same pattern as useSidebarDrawer — keeps the
+  // drawer from leaking between sessions when the user navigates.
+  useEffect(() => {
+    setOpen(false);
+  }, [location.pathname]);
+
+  const toggle = useCallback(() => setOpen((v) => !v), []);
+
+  return <Ctx.Provider value={{ open, setOpen, toggle }}>{children}</Ctx.Provider>;
+}
+
+export function useRightRailDrawer(): RightRailDrawerState {
+  const ctx = useContext(Ctx);
+  if (!ctx) {
+    // Soft fallback so consumers don't crash if rendered outside a provider.
+    return { open: false, setOpen: () => {}, toggle: () => {} };
+  }
+  return ctx;
+}