fix: Allow field-based sorting to persist in aggregate views

Fixed state.py sort validation that was resetting field sorts to AMOUNT:
- cycle_grouping() now allows MERCHANT/CATEGORY/GROUP/ACCOUNT sorts
- go_back() now allows field sorts when returning to aggregate views

This was preventing the 's' key from cycling to field sorts because
the view switching logic would immediately reset it back to AMOUNT.

Now field sorting works correctly in all aggregate views.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: wesm

moneyflow/state.py

@@ -270,7 +270,15 @@ def cycle_grouping(self) -> str:
         self.selected_account = None
 
         # Reset sort to valid field for aggregate views if needed
-        if self.sort_by not in [SortMode.COUNT, SortMode.AMOUNT]:
+        # Now includes field-based sorting (MERCHANT, CATEGORY, GROUP, ACCOUNT)
+        if self.sort_by not in [
+            SortMode.COUNT,
+            SortMode.AMOUNT,
+            SortMode.MERCHANT,
+            SortMode.CATEGORY,
+            SortMode.GROUP,
+            SortMode.ACCOUNT,
+        ]:
             self.sort_by = SortMode.AMOUNT
 
         # Cycle through views
@@ -412,8 +420,15 @@ def go_back(self) -> tuple[bool, int]:
 
             # Reset sort to valid field for aggregate views
             # (Detail views can have DATE, MERCHANT, CATEGORY, ACCOUNT, AMOUNT)
-            # (Aggregate views only have COUNT, AMOUNT)
-            if self.sort_by not in [SortMode.COUNT, SortMode.AMOUNT]:
+            # (Aggregate views have COUNT, AMOUNT, and field-based sorting)
+            if self.sort_by not in [
+                SortMode.COUNT,
+                SortMode.AMOUNT,
+                SortMode.MERCHANT,
+                SortMode.CATEGORY,
+                SortMode.GROUP,
+                SortMode.ACCOUNT,
+            ]:
                 self.sort_by = SortMode.AMOUNT
                 self.sort_direction = SortDirection.DESC