fix: Load past conversations using the most recent leaf (#1208)

[satyammistari]

Description:-
This PR fixes the issue where loading a past conversation with multiple branches would not always land the user on the most recently active branch.

Changes:-

• Modified createMessagesPath in +page.svelte to identify all leaf nodes (messages without children).
• Added logic to sort leaf nodes by updatedAt to find the most recent interaction point.
• Defaults the view to the latest leaf if no leafId is provided in the URL or LocalStorage.

How to Test:-

• Start a conversation and create a branch by editing an early message.
• Continue the new branch with 1-2 more messages.
• Navigate away and back to the conversation.
• Observe that the UI correctly restores the most recent branch instead of defaulting to the first/last message index.

Fixes #1208