Feature: Persist preview panel size as default

Feature Request

Describe the feature:
The preview panel (used for dev server previews) does not retain its size between sessions. Each time a new session starts, it resets to the default size and must be manually resized.

Expected behavior:
The preview panel width/size should be persisted and restored as the default on next session start — either via settings.json or internal UI state storage.

Current behavior:
Size resets every session. No setting in settings.json controls this.

Suggested approach:

• Add a preview.defaultWidth or similar field to settings.json, or
• Persist the last-used panel size in local UI state automatically

Use case:
Developers who consistently use the preview panel at a specific size have to resize it every session, which is unnecessary friction.