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>

tests/gui/test_smoke.py

@@ -52,15 +52,15 @@ PAGE_SLUGS = [
 # When a page gains real Spanish translation, flip its 'es' entry to
 # the localized substring — the test surface stays the same.
 EXPECTED_SUBSTRINGS: dict[str, dict[str, str]] = {
-    "1_Deduplicator":        {"en": "Find Duplicates",   "es": "Find Duplicates"},
-    "2_Text_Cleaner":        {"en": "Clean Text",        "es": "Clean Text"},
-    "3_Format_Standardizer": {"en": "Standardize",       "es": "Standardize"},
-    "4_Missing_Values":      {"en": "Fix Missing",       "es": "Fix Missing"},
-    "5_Column_Mapper":       {"en": "Map Columns",       "es": "Map Columns"},
-    "6_Outlier_Detector":    {"en": "Unusual",           "es": "Unusual"},
-    "7_Multi_File_Merger":   {"en": "Combine Files",     "es": "Combine Files"},
-    "8_Validator_Reporter":  {"en": "Quality Check",     "es": "Quality Check"},
-    "9_Pipeline_Runner":     {"en": "Automated",         "es": "Automated"},
+    "1_Deduplicator":        {"en": "Find Duplicates",   "es": "Buscar duplicados"},
+    "2_Text_Cleaner":        {"en": "Clean Text",        "es": "Limpiar texto"},
+    "3_Format_Standardizer": {"en": "Standardize",       "es": "Estandarizar"},
+    "4_Missing_Values":      {"en": "Fix Missing",       "es": "Corregir valores"},
+    "5_Column_Mapper":       {"en": "Map Columns",       "es": "Mapear columnas"},
+    "6_Outlier_Detector":    {"en": "Unusual",           "es": "atípicos"},
+    "7_Multi_File_Merger":   {"en": "Combine Files",     "es": "Combinar archivos"},
+    "8_Validator_Reporter":  {"en": "Quality Check",     "es": "Verificación de calidad"},
+    "9_Pipeline_Runner":     {"en": "Automated",         "es": "Flujos automatizados"},
     "99_Close":              {"en": "Shutting down",     "es": "Cerrando"},
 }