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/i18n/packs/en.json

@@ -106,39 +106,57 @@
   "tools": {
     "01_deduplicator": {
       "name": "Find Duplicates",
-      "description": "Fuzzy matching, normalization, survivor selection, and interactive review."
+      "description": "Fuzzy matching, normalization, survivor selection, and interactive review.",
+      "page_title": "🔍 Find Duplicates",
+      "page_caption": "Find and remove duplicate rows in CSV, delimited text, and Excel files."
     },
     "02_text_cleaner": {
       "name": "Clean Text",
-      "description": "Whitespace trim, multi-space collapse, Unicode normalization, BOM and line-ending handling."
+      "description": "Whitespace trim, multi-space collapse, Unicode normalization, BOM and line-ending handling.",
+      "page_title": "✂️ Clean Text",
+      "page_caption": "Trim whitespace, fold smart quotes, strip invisible characters, and normalize line endings. Runs locally — your data never leaves this computer."
     },
     "03_format_standardizer": {
       "name": "Standardize Formats",
-      "description": "Standardize dates, currencies, names, phone numbers, and addresses."
+      "description": "Standardize dates, currencies, names, phone numbers, and addresses.",
+      "page_title": "📐 Standardize Formats",
+      "page_caption": "Canonicalize dates, phone numbers, currency, names, addresses, and booleans on a per-column basis. Runs locally — your data never leaves this computer."
     },
     "04_missing_handler": {
       "name": "Fix Missing Values",
-      "description": "Detect disguised nulls, missingness analysis, and imputation strategies."
+      "description": "Detect disguised nulls, missingness analysis, and imputation strategies.",
+      "page_title": "🕳️ Fix Missing Values",
+      "page_caption": "Detect disguised nulls, profile missingness, and apply imputation or drop strategies. Runs locally — your data never leaves this computer."
     },
     "05_column_mapper": {
       "name": "Map Columns",
-      "description": "Rename columns, enforce a target schema, and coerce types."
+      "description": "Rename columns, enforce a target schema, and coerce types.",
+      "page_title": "🗂️ Map Columns",
+      "page_caption": "Rename columns, enforce a target schema, and coerce types. Runs locally — your data never leaves this computer."
     },
     "06_outlier_detector": {
       "name": "Find Unusual Values",
-      "description": "Z-score, IQR, and MAD detection with domain-rule violations and winsorization."
+      "description": "Z-score, IQR, and MAD detection with domain-rule violations and winsorization.",
+      "page_title": "📊 Find Unusual Values",
+      "page_caption": "Detect and handle outliers in numeric columns."
     },
     "07_multi_file_merger": {
       "name": "Combine Files",
-      "description": "Combine multiple CSV/Excel files with schema alignment."
+      "description": "Combine multiple CSV/Excel files with schema alignment.",
+      "page_title": "📎 Combine Files",
+      "page_caption": "Combine multiple CSV and Excel files into one dataset."
     },
     "08_validator_reporter": {
       "name": "Quality Check",
-      "description": "Validate against rules and generate PDF/Excel quality reports."
+      "description": "Validate against rules and generate PDF/Excel quality reports.",
+      "page_title": "✅ Quality Check",
+      "page_caption": "Validate data against rules and generate quality reports."
     },
     "09_pipeline_runner": {
       "name": "Automated Workflows",
-      "description": "Chain tools in recommended order and pass output between steps."
+      "description": "Chain tools in recommended order and pass output between steps.",
+      "page_title": "⚙️ Automated Workflows",
+      "page_caption": "Chain DataTools cleaning steps into one repeatable workflow. The pipeline recommends an order; you stay in control."
     }
   },
   "nav": {