feat: conversation folders and subfolders in sidebar

[andrewchumchal]

Summary

Add folder and subfolder support to the primary sidebar's conversation list — users can organise conversations into a nested tree structure, drag to reorder, and collapse/expand folders. Mirrors VS Code's file explorer.

Motivation

As conversation history grows, a flat list becomes unmanageable. Folders let users organise by project, topic, or workflow — e.g. a "Work" folder with sub-folders per client, or a "Research" folder grouped by topic.

Proposed behaviour

Data model

interface ConversationFolder {
  id: string;
  name: string;
  parentId: string | null; // null = root
  order: number;
  collapsed: boolean;
}

conversationStore gains:

• folders: ConversationFolder[]
• Each Conversation gains an optional folderId: string | null
• createFolder(name, parentId?), renameFolder, deleteFolder, moveConversation(id, folderId)

UI

• Sidebar conversation list renders as a tree
• Folders show a chevron (▶ / ▼) to collapse/expand
• Click chevron or folder name to toggle
• Conversations inside a folder are indented
• Drag-and-drop to move conversations into/out of folders and reorder
• Right-click context menu on folder: Rename, New subfolder, Delete
• Right-click on conversation: Move to folder, Duplicate, Delete
• + button in sidebar header: New Conversation | New Folder

Root level

• Conversations without a folder appear at the root (unfiled)
• "Unfiled" is not a real folder — just conversations with folderId: null

Collapsed state

• Collapsed folders show the folder name + conversation count badge
• Collapsed state persisted per folder

Acceptance criteria

• ConversationFolder model added to conversationStore
• Conversations can be assigned to folders
• Folder tree renders in the Chats sidebar panel
• Folders collapse/expand with state persisted
• Drag-and-drop to reorganise (at minimum: drag conversation into folder)
• Right-click context menus on folders and conversations
• Subfolder support (at least 2 levels deep)
• Folder structure survives app restarts

Prerequisites

• feat: app shell — activity bar + primary sidebar panel switching #14 App shell (Chats panel is part of the primary sidebar)