Fix #631: respect user and group default currency when creating expenses (#647)

* fix: respect user and group default currency when creating expenses

Closes #631

The expense form always defaulted to USD regardless of the user's or
group's configured default currency. The user/group defaults were
fetched but never applied to the form's currency field.

Changes:
- src/pages/add.tsx: set currency from user.defaultCurrency for
  non-group expenses, and from group.defaultCurrency (falling back to
  the user's) when entering a group context. The setCurrentUser effect
  is gated on router.isReady so the first render (before Next.js
  populates router.query) does not falsely treat a group page as a
  non-group page and apply the wrong default.
- src/store/addStore.ts: resetState now restores the user's default
  currency on unmount so navigating away and back lands on the correct
  default instead of stale state.

Disclaimer: parts of this change were drafted with the help of
Claude Opus 4.7.

* fix: apply full currency preference chain in add expense

Address review feedback on #647: prefer the user's last-used currency
(User.currency) before falling back to the group default and the user's
defaultCurrency setting. Also drop the currency override in resetState
so that resetting the form keeps the last-used currency.

Preference chain when initializing the add-expense page:
  1. currentUser.currency (last used)
  2. group.defaultCurrency (when in a group context)
  3. currentUser.defaultCurrency

Disclaimer: parts of this change were drafted with the help of Claude Opus 4.7.

* fix: keep last-used currency in store across remounts

When picking a currency, also update currentUser.currency in the
add-expense store so the preference chain on the next /add mount sees
the just-picked value without waiting for the async next-auth session
update to settle. On mount, the AddPage effect also preserves the
stored currency for the same user id instead of overwriting it with
potentially stale session data.

Disclaimer: parts of this change were drafted with the help of Claude Opus 4.7.

* fix: await session refresh before navigating after expense submit

Replace the previous remount-time currency preservation with a simpler
fix: in the expense submit handler, await the next-auth session update
(which writes the just-picked currency back into the session) BEFORE
navigating away. This guarantees that any subsequent /add mount reads
the up-to-date currentUser.currency without local-store overrides.

Reverts the in-store overwrite from 49b9778 so currentUser in the
add-expense store keeps mirroring the session user as-is.

Disclaimer: parts of this change were drafted with the help of Claude Opus 4.7.

Author: phof

src/components/AddExpense/AddExpensePage.tsx

@@ -172,17 +172,15 @@ export const AddOrEditExpensePage: React.FC<{
                   navPromise = async () => router.back();
                 }
 
-                navPromise()
+                update((session: any) => ({
+                  ...session,
+                  user: {
+                    ...(session?.user ?? {}),
+                    currency,
+                  },
+                }))
+                  .then(() => navPromise())
                   .then(() => resetState())
-                  .then(() =>
-                    update((session: any) => ({
-                      ...session,
-                      user: {
-                        ...(session?.user ?? {}),
-                        currency,
-                      },
-                    })),
-                  )
                   .catch(console.error);
               }
             }

src/pages/add.tsx

@@ -50,6 +50,9 @@ const AddPage: NextPageWithUser<{
   const { setMaxUploadFileSizeMB } = useAppStore((s) => s.actions);
   setMaxUploadFileSizeMB(maxUploadFileSizeMB);
 
+  const router = useRouter();
+  const { friendId, groupId, expenseId } = router.query;
+
   useEffect(() => {
     setCurrentUser({
       ...user,
@@ -61,10 +64,13 @@ const AddPage: NextPageWithUser<{
       obapiProviderId: user.obapiProviderId ?? null,
       bankingId: user.bankingId ?? null,
     });
-  }, [setCurrentUser, user]);
-
-  const router = useRouter();
-  const { friendId, groupId, expenseId } = router.query;
+    if (router.isReady && !groupId) {
+      const preferredCurrency = user.currency ?? user.defaultCurrency;
+      if (preferredCurrency) {
+        setCurrency(parseCurrencyCode(preferredCurrency));
+      }
+    }
+  }, [setCurrentUser, setCurrency, groupId, router.isReady, user]);
 
   const _groupId = parseInt(groupId as string);
   const _friendId = parseInt(friendId as string);
@@ -117,6 +123,11 @@ const AddPage: NextPageWithUser<{
           .map((gu) => gu.user)
           .filter((groupUser) => groupUser.id !== currentUser.id),
       ]);
+      const preferredCurrency =
+        currentUser.currency ?? groupQuery.data.defaultCurrency ?? currentUser.defaultCurrency;
+      if (preferredCurrency) {
+        setCurrency(parseCurrencyCode(preferredCurrency));
+      }
       const parsedDefaultSplit = deserializeDefaultSplit(groupQuery.data.defaultSplit);
       if (parsedDefaultSplit) {
         applySplitPreset(parsedDefaultSplit.splitType, parsedDefaultSplit.shares);
@@ -131,6 +142,7 @@ const AddPage: NextPageWithUser<{
     currentUser,
     setGroup,
     setParticipants,
+    setCurrency,
     applySplitPreset,
   ]);