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/pages/5_Column_Mapper.py

@@ -456,6 +456,8 @@ with dl_c:
         mime="application/json",
     )
 
+back_to_home_link(key="_back_to_home_link_bottom")
+
 st.divider()
 st.caption("Runs locally. Your data never leaves this computer. | DataTools v3.0")