feat: add envelopes (#2025)

This PR is handles the changes required to support envelopes. The new
envelope editor/signing page will be hidden during release.

The core changes here is to migrate the documents and templates model to
a centralized envelopes model.

Even though Documents and Templates are removed, from the user
perspective they will still exist as we remap envelopes to documents and
templates.

packages/lib/client-only/hooks/use-envelope-autosave.ts

@@ -0,0 +1,90 @@
+import { useCallback, useEffect, useRef, useState } from 'react';
+
+export function useEnvelopeAutosave<T>(saveFn: (data: T) => Promise<void>, delay = 1000) {
+  const timeoutRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+  const lastArgsRef = useRef<T | null>(null);
+  const pendingPromiseRef = useRef<Promise<void> | null>(null);
+
+  const [isPending, setIsPending] = useState(false);
+  const [isCommiting, setIsCommiting] = useState(false);
+
+  const triggerSave = useCallback(
+    (data: T) => {
+      lastArgsRef.current = data;
+
+      // A debounce or promise means something is pending
+      setIsPending(true);
+
+      if (timeoutRef.current) clearTimeout(timeoutRef.current);
+
+      // eslint-disable-next-line @typescript-eslint/no-misused-promises
+      timeoutRef.current = setTimeout(async () => {
+        if (!lastArgsRef.current) {
+          return;
+        }
+
+        const args = lastArgsRef.current;
+        lastArgsRef.current = null;
+        timeoutRef.current = null;
+
+        setIsCommiting(true);
+        pendingPromiseRef.current = saveFn(args);
+
+        try {
+          await pendingPromiseRef.current;
+        } finally {
+          // eslint-disable-next-line require-atomic-updates
+          pendingPromiseRef.current = null;
+          setIsCommiting(false);
+          setIsPending(false);
+        }
+      }, delay);
+    },
+    [saveFn, delay],
+  );
+
+  const flush = useCallback(async () => {
+    if (timeoutRef.current) {
+      clearTimeout(timeoutRef.current);
+      timeoutRef.current = null;
+    }
+
+    if (pendingPromiseRef.current) {
+      // Already running → wait for it
+      await pendingPromiseRef.current;
+      return;
+    }
+
+    if (lastArgsRef.current) {
+      const args = lastArgsRef.current;
+      lastArgsRef.current = null;
+
+      setIsCommiting(true);
+      setIsPending(true);
+
+      pendingPromiseRef.current = saveFn(args);
+      try {
+        await pendingPromiseRef.current;
+      } finally {
+        // eslint-disable-next-line require-atomic-updates
+        pendingPromiseRef.current = null;
+        setIsCommiting(false);
+        setIsPending(false);
+      }
+    }
+  }, [saveFn]);
+
+  useEffect(() => {
+    const handleBeforeUnload = () => {
+      if (timeoutRef.current || pendingPromiseRef.current) {
+        void flush();
+      }
+    };
+
+    window.addEventListener('beforeunload', handleBeforeUnload);
+
+    return () => window.removeEventListener('beforeunload', handleBeforeUnload);
+  }, [flush]);
+
+  return { triggerSave, flush, isPending, isCommiting };
+}