fix(nav): restore real Streamlit Back-to-Home button — preserves state

Reported: after the sticky-footer href fix (be7191a) the back-to-home
click worked but the home-page upload list disappeared. Full-page
navigation via ``<a href>`` doesn't preserve ``st.session_state`` on
the user's Streamlit build.

Trade-off forced: pick visible-from-anywhere sticky footer OR state
preservation. Can't have both because ``st.switch_page`` (soft nav,
preserves state) needs a real Streamlit button widget, and Streamlit
widgets can't be reliably CSS-positioned to the viewport bottom —
Streamlit owns the widget DOM and remounts it on every rerun.

State preservation wins. Going back to the pre-sticky design:

- ``render_sticky_footer()`` becomes a no-op shim. Kept as a callable
  so the call sites in every tool page don't have to be touched in
  this commit; the original implementation is preserved as
  ``_render_sticky_footer_DISABLED`` if we ever decide to revisit.
- Every Ready/Coming-Soon tool page (1-9) gets ``back_to_home_link()``
  reinstated near the top of the page (visible at scroll-top) AND
  ``back_to_home_link(key="_back_to_home_link_bottom")`` reinstated
  near the bottom of the page (visible at scroll-bottom). Both
  instances call ``st.switch_page`` via the existing helper — soft
  nav, no full reload, ``st.session_state["home_uploads"]`` and
  every other session-state key survive.

User trades the "always-visible while scrolling" sticky behavior for
the upload-list-survives-navigation behavior. The two-button pattern
(top + bottom) was what we had before the sticky-footer experiment;
on short pages both are visible at once, on long pages the user has
one in reach at either end.

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

src/gui/components/_legacy.py

@@ -521,6 +521,32 @@ html_download_button = local_download_button
 
 
 def render_sticky_footer() -> None:
+    """No-op shim.
+
+    The previous implementation injected a CSS-positioned ``<a href>``
+    footer into the page body. It looked nice (always-visible bar at
+    the viewport bottom) but the click was a FULL-page navigation,
+    which on the user's Streamlit build doesn't preserve
+    ``st.session_state`` — uploads disappeared after every Back to
+    Home click. Soft navigation via ``st.switch_page`` requires a
+    real Streamlit button widget, which we can't reliably
+    CSS-position to the viewport bottom because Streamlit owns the
+    widget's DOM container and remounts it on every rerun.
+
+    So the sticky footer is retired. Each tool page renders the
+    real Streamlit-button version of ``back_to_home_link`` near the
+    top AND near the bottom (the two-instance pattern from before
+    the sticky-footer attempt), so the button is visible at both
+    ends of the page without ever risking state loss.
+
+    Kept as a callable so existing tool-page imports + call sites
+    don't have to be touched in this commit — they all resolve to
+    this no-op.
+    """
+    return
+
+
+def _render_sticky_footer_DISABLED() -> None:
     """Slim fixed-position footer at the bottom of the viewport.
 
     Contains a "Back to Home" link that's always visible regardless of