server : (webui) introduce conversation branching + idb storage

[ngxson]

In this PR:

• Add conversation "branching", allow user to keep the old conversation history when edit / regenerate a message ==> Implemented using node-based system, which is the same as ChatGPT and Hugging Chat
• Add IndexedDB as main storage, powered by dexie library
• Auto migration from localStorage to IndexedDB upon running the first time

Auto-migration

Upon launching this new version, the auto migration will be triggered automatically. This will copy all conversations from localStorage to IndexedDB. Please note that:

• Old conversations are not deleted from localStorage
• A new item named migratedToIDB will be stored in localStorage to mark that the migration has already been done, preventing duplicated migration

How it works

The content below is copied from types.ts

What is conversation "branching"? It is a feature that allows the user to edit an old message in the history, while still keeping the conversation flow.
Inspired by ChatGPT / Claude / Hugging Chat where you edit a message, a new branch of the conversation is created, and the old message is still visible.

We use the same node-based structure like other chat UIs, where each message has a parent and children. A "root" message is the first message in a conversation, which will not be displayed in the UI.

 root
  ├── message 1
  │      └── message 2
  │             └── message 3
  └── message 4
        └── message 5

In the above example, assuming that user wants to edit message 2, a new branch will be created:

          ├── message 2
          │   └── message 3
          └── message 6

Message 2 and 6 are siblings, and message 6 is the new branch.

We only need to know the last node (aka leaf) to get the current branch. In the above example, message 5 is the leaf of branch containing message 4 and 5.

For the implementation:

• StorageUtils.getMessages() returns list of all nodes
• StorageUtils.filterByLeafNodeId() filters the list of nodes from a given leaf node