fix(nav): switch_page resolves correctly + bottom-of-page back link

Two issues, same fix surface.

(1) Reported crash on Back-to-Home:

    StreamlitAPIException: Could not find page: app.py.

``st.switch_page("app.py")`` doesn't work under ``st.navigation`` —
the entry script is the nav manager itself and is not a registered
page. The fix needs to pass an ``st.Page`` object whose script
identity matches one registered in the nav.

First-pass attempt (``from src.gui.app import _home_page``) hit a
worse failure: importing ``app.py`` from inside a tool-page render
re-executes the nav setup with the WRONG "main script" context, so
every ``st.Page("pages/N_foo.py", ...)`` call in ``_build_navigation``
fails with "file could not be found".

Extract the home renderer into its own module ``src/gui/_home.py``
which has no top-level Streamlit side effects. Both the nav manager
and the back-link helper import ``_home_page`` from there. The Page
object built at click time has the same callable identity as the one
registered, so ``st.switch_page`` resolves it.

(2) Reported UX: the back button scrolled out of view on long pages.

Add a second ``back_to_home_link(key="_back_to_home_link_bottom")``
call near the footer of every tool page (1-9). The unique key avoids
widget-id collision with the top instance. Coming-Soon stubs get it
unconditionally; Ready tools render it only after a result exists
because the page short-circuits with ``st.stop()`` before then —
when no result is on screen the page is short enough that the top
link is sufficient.

2220 tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/gui/components/_legacy.py

@@ -410,22 +410,33 @@ html_download_button = local_download_button
 
 
 def back_to_home_link(*, key: str = "_back_to_home_link") -> None:
-    """Render a small "← Back to Home" affordance near the top of a tool page.
+    """Render a "← Back to Home" affordance on a tool page.
 
     Tool pages reached from the home findings panel benefit from an
     explicit return-to-home control so a user working through findings
     on multiple uploaded files can hop between files without hunting
-    through the sidebar.
+    through the sidebar. Call this twice on each tool page — once
+    near the top (default key) and once at the bottom with
+    ``key="_back_to_home_link_bottom"`` so the control stays reachable
+    after the user scrolls through long results.
 
-    Implementation note: ``st.switch_page("app.py")`` routes back to the
-    entry script which, under ``st.navigation``, lands on the default
-    page (Home). Streamlit's button is used (rather than ``st.page_link``)
-    because the entry script is a navigation manager, not a registered
-    Page object, and ``page_link`` to ``app.py`` renders inconsistently
-    across Streamlit minor versions.
+    Implementation: ``st.switch_page`` under ``st.navigation`` requires
+    either a file path to a page in ``pages/`` or a ``StreamlitPage``
+    object whose script identity matches one registered in the nav.
+    The entry script ``app.py`` is the nav manager itself — it cannot
+    be switched-to by filename. So we import the home callable from
+    ``src.gui.app`` and rebuild the same ``st.Page`` registration here.
+    Streamlit identifies pages by the underlying callable's qualified
+    name, so a freshly-constructed Page resolves to the registered one.
     """
     if st.button(_t("nav.back_to_home"), key=key, type="secondary"):
-        st.switch_page("app.py")
+        # Import from the renderer module (not from app.py — importing
+        # app.py would re-execute its navigation setup with the wrong
+        # "main script" context and blow up the pages/ path resolution).
+        from src.gui._home import _home_page
+        st.switch_page(
+            st.Page(_home_page, title="Home", icon="🧹", url_path="home"),
+        )
 
 
 def shutdown_app() -> None: