fix: Prevent IndexError when navigating during async archive operations

Clamp current_index to valid range in find_prev_valid_id and
find_next_valid_id to handle race condition where user navigates
while envelope list is being refreshed after archiving.

src/mail/message_store.py

@@ -83,7 +83,17 @@ class MessageStore:
         self, current_index: int
     ) -> Tuple[Optional[int], Optional[int]]:
         """Find the next valid message ID and its index"""
-        if not self.envelopes or current_index >= len(self.envelopes) - 1:
+        if not self.envelopes:
+            return None, None
+
+        # Clamp current_index to valid range in case list shrunk during async operations
+        if current_index >= len(self.envelopes):
+            current_index = len(self.envelopes) - 1
+        if current_index < 0:
+            current_index = 0
+
+        # If already at or past the end, no next item
+        if current_index >= len(self.envelopes) - 1:
             return None, None
 
         # Start from current index + 1
@@ -99,7 +109,17 @@ class MessageStore:
         self, current_index: int
     ) -> Tuple[Optional[int], Optional[int]]:
         """Find the previous valid message ID and its index"""
-        if not self.envelopes or current_index <= 0:
+        if not self.envelopes:
+            return None, None
+
+        # Clamp current_index to valid range in case list shrunk during async operations
+        if current_index >= len(self.envelopes):
+            current_index = len(self.envelopes) - 1
+        if current_index < 0:
+            current_index = 0
+
+        # If at the beginning, no previous item
+        if current_index <= 0:
             return None, None
 
         # Start from current index - 1