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

src/gui/pages/1_Deduplicator.py

@@ -33,6 +33,7 @@ from src.license import FeatureFlag
 
 hide_streamlit_chrome()
 render_sticky_footer()
+back_to_home_link()
 from src.audit import log_page_open
 log_page_open("1_Deduplicator")
 require_feature_or_render_upgrade(FeatureFlag.DEDUPLICATOR)
@@ -405,6 +406,8 @@ else:
 # Footer
 # ---------------------------------------------------------------------------
 
+back_to_home_link(key="_back_to_home_link_bottom")
+
 st.divider()
 st.caption(
     "Runs locally. Your data never leaves this computer. "

src/gui/pages/2_Text_Cleaner.py

@@ -35,6 +35,7 @@ from src.core.text_clean import (
 
 hide_streamlit_chrome()
 render_sticky_footer()
+back_to_home_link()
 from src.audit import log_page_open
 log_page_open("2_Text_Cleaner")
 require_feature_or_render_upgrade(FeatureFlag.TEXT_CLEANER)
@@ -382,6 +383,8 @@ 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")
 

src/gui/pages/3_Format_Standardizer.py

@@ -33,6 +33,7 @@ from src.license import FeatureFlag
 
 hide_streamlit_chrome()
 render_sticky_footer()
+back_to_home_link()
 from src.audit import log_page_open
 log_page_open("3_Format_Standardizer")
 require_feature_or_render_upgrade(FeatureFlag.FORMAT_STANDARDIZER)
@@ -652,6 +653,8 @@ 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")
 

src/gui/pages/4_Missing_Values.py

@@ -34,6 +34,7 @@ from src.license import FeatureFlag
 
 hide_streamlit_chrome()
 render_sticky_footer()
+back_to_home_link()
 from src.audit import log_page_open
 log_page_open("4_Missing_Values")
 require_feature_or_render_upgrade(FeatureFlag.MISSING_HANDLER)
@@ -415,6 +416,8 @@ 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")
 

src/gui/pages/5_Column_Mapper.py

@@ -35,6 +35,7 @@ from src.license import FeatureFlag
 
 hide_streamlit_chrome()
 render_sticky_footer()
+back_to_home_link()
 from src.audit import log_page_open
 log_page_open("5_Column_Mapper")
 require_feature_or_render_upgrade(FeatureFlag.COLUMN_MAPPER)
@@ -459,6 +460,8 @@ 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")
 

src/gui/pages/6_Outlier_Detector.py

@@ -22,6 +22,7 @@ from src.license import FeatureFlag
 
 hide_streamlit_chrome()
 render_sticky_footer()
+back_to_home_link()
 from src.audit import log_page_open
 log_page_open("6_Outlier_Detector")
 require_feature_or_render_upgrade(FeatureFlag.OUTLIER_DETECTOR)
@@ -100,6 +101,8 @@ st.button("Detect Outliers", type="primary", use_container_width=True, disabled=
 # Footer
 # ---------------------------------------------------------------------------
 
+back_to_home_link(key="_back_to_home_link_bottom")
+
 st.divider()
 st.caption(
     "Runs locally. Your data never leaves this computer. "

src/gui/pages/7_Multi_File_Merger.py

@@ -22,6 +22,7 @@ from src.license import FeatureFlag
 
 hide_streamlit_chrome()
 render_sticky_footer()
+back_to_home_link()
 from src.audit import log_page_open
 log_page_open("7_Multi_File_Merger")
 require_feature_or_render_upgrade(FeatureFlag.MULTI_FILE_MERGER)
@@ -98,6 +99,8 @@ st.button("Merge Files", type="primary", use_container_width=True, disabled=True
 # Footer
 # ---------------------------------------------------------------------------
 
+back_to_home_link(key="_back_to_home_link_bottom")
+
 st.divider()
 st.caption(
     "Runs locally. Your data never leaves this computer. "

src/gui/pages/8_Validator_Reporter.py

@@ -22,6 +22,7 @@ from src.license import FeatureFlag
 
 hide_streamlit_chrome()
 render_sticky_footer()
+back_to_home_link()
 from src.audit import log_page_open
 log_page_open("8_Validator_Reporter")
 require_feature_or_render_upgrade(FeatureFlag.VALIDATOR_REPORTER)
@@ -105,6 +106,8 @@ st.button("Validate & Generate Report", type="primary", use_container_width=True
 # Footer
 # ---------------------------------------------------------------------------
 
+back_to_home_link(key="_back_to_home_link_bottom")
+
 st.divider()
 st.caption(
     "Runs locally. Your data never leaves this computer. "

src/gui/pages/9_Pipeline_Runner.py

@@ -36,6 +36,7 @@ from src.license import FeatureFlag
 
 hide_streamlit_chrome()
 render_sticky_footer()
+back_to_home_link()
 from src.audit import log_page_open
 log_page_open("9_Pipeline_Runner")
 require_feature_or_render_upgrade(FeatureFlag.PIPELINE_RUNNER)
@@ -417,6 +418,8 @@ 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")