docs: clean up Alpha 3 section, add Known Issues

Remove RevealJS thumbnails (redundant with Features section) and Upload
reliability (still unresolved). Add Known Issues section documenting
the large file upload timeout.

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

README.md

@@ -144,9 +144,6 @@ Refinements to the Typst editor and the format badge system:
   complete UI (login, dashboard, editor, settings, emails).
 - **Visual editors for Quarto and Typst** — bold, italic, headings and inline code
   shortcuts in the toolbar for `.qmd` and `.typ` files.
-- **RevealJS thumbnails** — first-slide PNG preview rendered in project cards.
-- **Upload reliability** — streaming response fix so large file uploads on slow
-  connections no longer time out at the proxy layer.
 - **Top/bottom split view** — new editor layout that stacks the source editor
   above the PDF preview vertically, in addition to the existing side-by-side mode.
 - **Bidirectional format export** — LaTeX projects can be exported to Typst,
@@ -157,6 +154,13 @@ Refinements to the Typst editor and the format badge system:
 
 ---
 
+## Known issues
+
+- **Large file upload timeouts** — uploads of large files on slow connections
+  can time out at the proxy layer. A streaming response fix is pending.
+
+---
+
 ## Security model — trusted environments only
 
 > [!CAUTION]