fix(web): pin preview header close button to the top-right corner

PanelHeader in wrap mode placed the Close control inside the same
wrapping flex row as the FilePreview actions, so it drifted to wherever
the last action landed as the panel width changed. Move title, subtitle
and slot content into a __main region that wraps on its own; Close stays
a sibling pinned to the corner at every width.

Remove the FilePreview download link; the downloadUrl prop stays because
it still feeds the PDF viewer source.

Author: elkaix

.changeset/web-preview-header-close.md

@@ -0,0 +1,5 @@
+---
+'@pymodel/pythinker-code': patch
+---
+
+Keep the file preview close button in the top-right corner at every panel width and remove the unused download action.

apps/pythinker-code/dist-web/.web-bundle-manifest.json

@@ -1,4 +1,4 @@
 {
-  "sourceHash": "426c0f6fb43b8b89ffb8ec4742cd3a4c735858eb57c2a81c31c6cc9ddfe74e70",
+  "sourceHash": "7ea5e8fb9bbb40d695d221dccebe6a9c77a67434b48c9ecfff3985148d67e43b",
   "sourceFileCount": 405
 }