feat(nav,i18n): sticky footer with Back-to-Home + localized tool headers

Two unrelated UX issues addressed in one sweep across all nine tool
pages because they share the same edit surface.

(1) Sticky footer replaces the top + bottom back-link buttons.

Reported: a big white empty footer space at the bottom of every page;
the Back to Home button at the top scrolled out of view on long pages.

New ``render_sticky_footer()`` helper in ``components/_legacy.py``
injects a fixed-position bar at ``bottom: 0`` of the viewport with:

- A border-top so it visually reads as a non-movable bar.
- A semi-transparent background (rgba 0.96 + ``backdrop-filter: blur``)
  so content underneath shows through faintly when the user scrolls.
- A styled ``<a href="home">`` anchor (not an ``st.button``) because
  Streamlit widgets can't be CSS-positioned reliably — Streamlit owns
  the widget's DOM container and re-mounts it on every rerun. A real
  anchor sits exactly where the CSS puts it and triggers Streamlit's
  URL routing to the home page.
- ``padding-bottom: 3.5rem`` on the main container so the last widget
  isn't hidden behind the bar.

Called once per tool page, immediately after ``hide_streamlit_chrome()``
so it renders even on pages that ``st.stop()`` early before any other
content runs. The old top-and-bottom ``back_to_home_link()`` calls are
removed from every tool page; their entry/exit points were dropping
the button when the script short-circuited.

(2) Tool-page headers now localize.

Reported: switching the sidebar language picker to Spanish left the
tool page's title + caption in English. Root cause: every page had
hard-coded ``st.title("✂️ Clean Text")`` / ``st.caption("Trim
whitespace...")`` strings.

Added per-tool ``tools.<id>.page_title`` and
``tools.<id>.page_caption`` keys to ``en.json`` and ``es.json`` for
all nine tools. Routed each page's title/caption call through ``t()``.
Verified: with ``ui_lang=es`` set, the Clean Text page now renders
"✂️ Limpiar texto" + the Spanish caption.

Updated ``tests/gui/test_smoke.py::EXPECTED_SUBSTRINGS`` so the
``es`` column for each tool page asserts the actual Spanish string
(was a duplicate of the English string back when the page bodies
were English-only).

2220 tests pass.

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

src/gui/pages/7_Multi_File_Merger.py

@@ -13,21 +13,23 @@ if str(_project_root) not in sys.path:
 
 from src.gui.components import (
     back_to_home_link,
+    render_sticky_footer,
     hide_streamlit_chrome,
     require_feature_or_render_upgrade,
 )
+from src.i18n import t
 from src.license import FeatureFlag
 
 hide_streamlit_chrome()
-back_to_home_link()
+render_sticky_footer()
 require_feature_or_render_upgrade(FeatureFlag.MULTI_FILE_MERGER)
 
 # ---------------------------------------------------------------------------
 # Header
 # ---------------------------------------------------------------------------
 
-st.title("📎 Combine Files")
-st.caption("Combine multiple CSV and Excel files into one dataset.")
+st.title(t("tools.07_multi_file_merger.page_title"))
+st.caption(t("tools.07_multi_file_merger.page_caption"))
 
 st.info("This tool is under development.")
 
@@ -94,10 +96,9 @@ 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. "
     "| DataTools v3.0"
 )
+