fix: configure Image extension as block-level to match Docmost's schema

The Image extension was configured with `inline: true`, but Docmost's
actual editor defines images as block-level nodes (`inline: false,
group: "block"`). This mismatch caused `update_page` to silently drop
all image nodes during the Yjs merge, since inline image nodes inside
paragraphs don't match Docmost's expected block-level image schema.

Fixes #4

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/tiptap-extensions.ts

@@ -11,7 +11,7 @@ export const tiptapExtensions = [
     heading: {},
   }),
   Image.configure({
-    inline: true,
+    inline: false,
   }),
   Link.configure({
     openOnClick: false,