fix: resolve readonly property error in assistant preset settings (#11491)

When updating assistant preset settings, if agent.settings was undefined,
it was assigned the DEFAULT_ASSISTANT_SETTINGS object directly. Since this
object is defined with `as const`, it is readonly and subsequent property
assignments would fail with "Cannot assign to read only property".

Fixed by creating a shallow copy of DEFAULT_ASSISTANT_SETTINGS instead of
referencing it directly.

Closes #11490

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

src/renderer/src/store/assistants.ts

@@ -216,7 +216,7 @@ const assistantsSlice = createSlice({
         if (agent.id === action.payload.assistantId) {
           for (const key in settings) {
             if (!agent.settings) {
-              agent.settings = DEFAULT_ASSISTANT_SETTINGS
+              agent.settings = { ...DEFAULT_ASSISTANT_SETTINGS }
             }
             agent.settings[key] = settings[key]
           }