feat: auto-focus editor on open and re-reveal

[LeonardoRick]

Summary

Currently when the markdown editor opens, the cursor doesn't appear in the editable area — users have to click inside to start typing.

This PR adds automatic focus in two scenarios:

1. First open: calls vditor.focus() in the Vditor after callback so the cursor appears immediately when the editor initializes
2. Re-reveal: when createOrShow is called on an already-open panel (e.g. via command palette), sends a focus message to the webview so the cursor reappears without requiring a click

Changes

• media-src/src/main.ts: added vditor.focus() in the after() callback and a focus message handler
• src/extension.ts: sends { command: 'focus' } to the webview after reveal()