fix(app): restore Home navigation as labelled "Problem" page link

Hiding the auto-generated "streamlit app" entry-page link in the sidebar
nav (commit b519ba2) removed the only way back to the problem-selection
page from sub-pages: users could no longer pick a problem after
navigating to Solve / Visualize / Compare.

Add an explicitly labelled `st.page_link("streamlit_app.py", label="Problem",
icon="🧩")` inside `sidebar_brand()` so every page gets a discoverable,
clickable link back to Home with a clear name. The CSS that hides the
auto-generated link stays in place so the navigator now shows
"🧩 Problem · Solve · Visualize · Compare" instead of the jargon
"streamlit app".

Author: Yuma Ichikawa

app/_common.py

@@ -759,6 +759,16 @@ def sidebar_brand() -> None:
             """,
             unsafe_allow_html=True,
         )
+        # The auto-generated entry-page link in Streamlit's multipage
+        # navigator gets its label from the file basename ("streamlit
+        # app"), which is jargony and confused users into thinking the
+        # page wasn't clickable. We hide that auto link in CSS and
+        # instead emit our own properly labelled link here so the user
+        # always has a discoverable way back to the problem-selection
+        # page.
+        if hasattr(st, "page_link"):
+            with contextlib.suppress(Exception):
+                st.page_link("streamlit_app.py", label="Problem", icon="🧩")
 
 
 def empty_state_card(