[NAH-30] loading spinners on every model call + button locking (#12)

Wrap deck-generation, grading, and next/recap in show_progress="full" so
a slow model call always shows a spinner — never a dead screen. Each button
is disabled and relabelled (Generating…/Grading…) while its handler runs,
then restored, preventing double-submits. Empty/error states from NAH-23
already surface friendly messages on the upload screen.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

app.py

@@ -145,18 +145,34 @@ with gr.Blocks(title="Recall — your AI study partner", theme=gr.themes.Soft())
     with gr.Accordion("🔍 Debug: extraction & chunks", open=False, visible=False) as debug_accordion:
         debug_data = gr.JSON(label="Content pipeline output")
 
+    # Every handler that can hit the model shows a spinner (show_progress="full")
+    # and disables its button while working, so a slow call never reads as a dead
+    # or double-clickable screen.
     start_btn.click(
+        lambda: gr.update(value="⏳ Generating deck…", interactive=False), None, start_btn,
+    ).then(
         start_session, [file_in, text_in],
         [session, upload_view, study_view, upload_msg, progress, question,
          debug_accordion, debug_data],
+        show_progress="full",
+    ).then(
+        lambda: gr.update(value="Generate deck & start", interactive=True), None, start_btn,
     )
+
     submit_btn.click(
+        lambda: gr.update(value="⏳ Grading…", interactive=False), None, submit_btn,
+    ).then(
         submit_answer, [session, answer_in],
         [session, verdict, followup, progress, next_btn],
+        show_progress="full",
+    ).then(
+        lambda: gr.update(value="Submit", interactive=True), None, submit_btn,
     )
+
     next_btn.click(
         show_next, [session],
         [session, question, study_view, recap_view, recap_md],
+        show_progress="full",
     ).then(lambda: ("", "", ""), None, [answer_in, verdict, followup])