Fix zap race condition

If zapping too fast, item.meSats inside optimistic update could include pending sats from previous zap unlike item.meSats in calling context.
This is fixed by passing satsDelta instead of calculating it independently with readFragment.
This race condition lead to following unexpected behavior that is now fixed:

* pending sats could go negative causing items to falsely be marked as flagged

unrelated fix:

* fix zapUndoTrigger using total sats not satsDelta

components/item-act.js

@@ -229,23 +229,7 @@ export function useAct () {
   return act
 }
 
-const zapOptimisticUpdate = (cache, { id, path, act, sats }, { me }) => {
-  const readItemFragment = id => cache.readFragment({
-    id: `Item:${id}`,
-    fragment: gql`
-        fragment ItemMeSatsZap on Item {
-          meSats
-        }
-      `
-  })
-
-  // determine how much we increased existing sats
-  // by checking the difference between result sats and meSats
-  // if it's negative, skip the cache as it's an out of order update
-  // if it's positive, add it to sats and commentSats
-  const item = readItemFragment(id)
-  const satsDelta = sats - item.meSats
-
+const zapOptimisticUpdate = (cache, { id, path, act, satsDelta }, { me }) => {
   if (satsDelta > 0) {
     updateItemSats(cache, { id, path, act, sats: satsDelta }, { me })
   }
@@ -282,24 +266,27 @@ export function useZap () {
 
     // add current sats to next tip since idempotent zaps use desired total zap not difference
     const sats = meSats + nextTip(meSats, { ...me?.privates })
+    const satsDelta = sats - meSats
 
     const act = 'TIP'
     let cancel, revert, nid
     try {
-      const variables = { path: item.path, id: item.id, sats, act }
-      revert = zapOptimisticUpdate(cache, variables, { me })
+      const variables = { id: item.id, path: item.path, act, sats }
+      revert = zapOptimisticUpdate(cache, { ...variables, satsDelta }, { me })
       strike()
       abortSignal?.start()
-      nid = notify(NotificationType.ZapPending, { amount: sats - meSats, itemId: item.id }, false)
-      if (zapUndoTrigger(me, sats)) {
+      nid = notify(NotificationType.ZapPending, { amount: satsDelta, itemId: item.id }, false)
+      if (zapUndoTrigger(me, satsDelta)) {
         await zapUndo(abortSignal)
       } else {
         abortSignal?.done()
       }
       let hash, hmac;
-      [{ hash, hmac }, cancel] = await payment.request(sats - meSats)
+      [{ hash, hmac }, cancel] = await payment.request(satsDelta)
       await zap({ variables: { ...variables, hash, hmac } })
-      if (me) persistItemPendingSats({ ...item, act, sats: -(sats - meSats) })
+      // in the success case, we need to remove pending sats
+      // since they will now be included in item.meSats of server responses
+      if (me && satsDelta > 0) persistItemPendingSats({ ...item, act, sats: -satsDelta })
     } catch (error) {
       revert?.()
       if (error instanceof InvoiceCanceledError || error instanceof ActCanceledError) {