refactor: extract ToolCallCard and ChatLobby components (#502)

* refactor: extract ToolCallCard and ChatLobby components

- Extract ToolCallCard.tsx (279 lines) for tool call UI rendering
- Extract ChatLobby.tsx (272 lines) for empty state with session history
- Reduce chat-message-display.tsx from 1760 to 1307 lines (-26%)

* fix: address PR review feedback

- Remove redundant key prop in ToolCallCard
- Make onDeleteSession optional and conditionally render delete button
- Extract shared types (DiagramOperation, ToolPartLike) to types.ts

components/chat/types.ts

@@ -0,0 +1,16 @@
+export interface DiagramOperation {
+    operation: "update" | "add" | "delete"
+    cell_id: string
+    new_xml?: string
+}
+
+export interface ToolPartLike {
+    type: string
+    toolCallId: string
+    state?: string
+    input?: {
+        xml?: string
+        operations?: DiagramOperation[]
+    } & Record<string, unknown>
+    output?: string
+}