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/components/_legacy.py

@@ -491,6 +491,88 @@ def local_download_button(
 html_download_button = local_download_button
 
 
+def render_sticky_footer() -> None:
+    """Render a slim fixed-position footer at the bottom of the viewport.
+
+    Contains a "Back to Home" link that's always visible regardless of
+    scroll position. Replaces the previous top + bottom-of-page
+    ``back_to_home_link`` buttons: a single sticky bar is more
+    discoverable, doesn't scroll out of view, and visually reads as a
+    "non-movable footer" (border-top + slim padding) rather than
+    just-another-button-at-the-bottom.
+
+    Implementation notes:
+
+    - Uses a styled ``<a href="home">`` anchor rather than an
+      ``st.button`` because Streamlit widgets can't be CSS-positioned
+      reliably (their DOM container is owned by Streamlit's renderer
+      and gets re-mounted on every rerun). A real anchor sits anywhere
+      we want and triggers Streamlit's URL routing to the home page.
+    - ``href="home"`` is a relative path so it works behind a reverse
+      proxy / non-root mount.
+    - We bump ``padding-bottom`` on the main container so the last
+      widget isn't hidden behind the fixed bar.
+    - Theme: ``rgba(255,255,255,0.96)`` background with a thin
+      semi-transparent border-top. The blur backdrop keeps content
+      slightly visible underneath. Works against both light and dark
+      Streamlit themes — explicit colours, not theme variables,
+      because Streamlit's CSS variable surface is unstable across
+      minor versions.
+    """
+    import html as _html
+    label = _html.escape(_t("nav.back_to_home"))
+    st.markdown(
+        f"""
+<style>
+[data-testid="stAppViewBlockContainer"] {{
+    padding-bottom: 3.5rem !important;
+}}
+.datatools-sticky-footer {{
+    position: fixed;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    background: rgba(255, 255, 255, 0.96);
+    backdrop-filter: blur(8px);
+    -webkit-backdrop-filter: blur(8px);
+    border-top: 1px solid rgba(49, 51, 63, 0.22);
+    padding: 0.45rem 1.25rem;
+    z-index: 9999;
+    display: flex;
+    align-items: center;
+    justify-content: flex-start;
+    font-family: inherit;
+}}
+.datatools-sticky-footer a {{
+    display: inline-block;
+    color: rgb(38, 39, 48);
+    text-decoration: none;
+    padding: 0.3rem 0.85rem;
+    border-radius: 0.5rem;
+    border: 1px solid rgba(49, 51, 63, 0.22);
+    background: rgb(240, 242, 246);
+    font-size: 14px;
+    font-weight: 500;
+    line-height: 1.4;
+    cursor: pointer;
+    transition: background 0.12s ease, border-color 0.12s ease;
+}}
+.datatools-sticky-footer a:hover {{
+    background: rgb(225, 228, 235);
+    border-color: rgba(49, 51, 63, 0.35);
+}}
+.datatools-sticky-footer a:active {{
+    background: rgb(210, 214, 222);
+}}
+</style>
+<div class="datatools-sticky-footer">
+    <a href="home" target="_self">{label}</a>
+</div>
+""",
+        unsafe_allow_html=True,
+    )
+
+
 def back_to_home_link(*, key: str = "_back_to_home_link") -> None:
     """Render a "← Back to Home" affordance on a tool page.