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.

styles.css

@@ -303,6 +303,44 @@
   border-color: var(--interactive-accent);
 }
 
+/* Chat History Selector */
+.ollama-history-selector {
+  padding: var(--size-4-1) var(--size-4-2);
+  border-radius: var(--ollama-radius);
+  border: 1px solid var(--ollama-border);
+  background-color: var(--background-modifier-form-field);
+  color: var(--text-normal);
+  font-size: var(--font-ui-small);
+  cursor: pointer;
+  max-width: 12rem;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
+.ollama-history-selector:focus {
+  outline: none;
+  border-color: var(--interactive-accent);
+}
+
+.ollama-history-delete-button {
+  display: inline-flex;
+  align-items: center;
+  gap: var(--size-4-1);
+  padding: var(--size-4-1) var(--size-4-2);
+  font-size: var(--font-smallest);
+  border-radius: var(--ollama-radius);
+  border: 1px solid var(--ollama-border);
+  background-color: var(--background-modifier-form-field);
+  color: var(--text-normal);
+  cursor: pointer;
+  transition: background-color 0.15s ease;
+}
+
+.ollama-history-delete-button:hover {
+  background-color: var(--background-modifier-error-hover);
+  color: var(--text-error);
+}
+
 /* Show Logs Button */
 .ollama-show-logs-button {
   display: inline-flex;