Add chat session history with dropdown UI

Implement persistent chat history using a new ChatHistoryManager class.
Sessions are saved on view close, restored on open, and selectable via a
dropdown in the chat header. Includes delete and clear-all commands,
with automatic title generation from the first user message.

tests/chat-view.test.ts

@@ -1,6 +1,7 @@
 import { ChatView } from '../src/chat-view';
 import { PluginSettings, OllamaMessage, ChatMessage, OllamaTool, ToolCall } from '../src/types';
 import { ErrorHandler } from '../src/error-handler';
+import { ChatHistoryManager } from '../src/chat-history';
 
 // Mock Obsidian types
 interface MockVault {
@@ -114,7 +115,14 @@ describe('ChatView', () => {
       app: mockApp,
     };
 
-    view = new ChatView(mockLeaf as unknown as any, mockSettings);
+    view = new ChatView(
+      mockLeaf as unknown as any,
+      mockSettings,
+      undefined,
+      undefined,
+      undefined,
+      new ChatHistoryManager()
+    );
     // Obsidian's contentEl has a createEl helper that standard DOM lacks
     // Unlike standard DOM, Obsidian elements can create nested elements with createEl
     const contentDiv = document.createElement('div') as any;