feat(home,sidebar): brand hero + sidebar = footer style + PNG icon

Bundles a handful of UX cleanups:

- Findings-card chevron moved to the LEFT side of the head. CSS still
  rotates it 90° between collapsed/expanded states.

- Tool-link buttons in findings rows (``Clean Text →`` etc.) are now
  left-justified against the icon column with minimal surrounding
  whitespace. Action column ratio dropped from 1.8 → 1.4 and the
  button switched from ``width="stretch"`` (centered text) to
  ``width="content"`` (shrinks to fit, left-aligned within column).

- Home-page hero now mirrors the sidebar brand block: 56px ink "D"
  chip on the left + "UNALOGIX" eyebrow stacked above "DataTools"
  wordmark, then the "Clean. Normalize. Transform." tagline beneath.
  New ``.dt-page-brand / -row / -words / -mark / -eyebrow /
  -wordmark`` rules in ``_DESIGN_TOKENS_CSS``. Streamlit wraps h1
  elements in an emotion-cache div with extra padding; a descendant
  flattener (``.dt-page-brand-words *`` margin:0 / padding:0) keeps
  the eyebrow + wordmark stack the same height as the chip so they
  center-align cleanly.

- Sidebar nav restyled to match the sticky-footer Help/Close buttons
  exactly: 13px / 500 / 1.3 line-height, 5×10px padding, 8px gap
  between icon and label, transparent background. Active item gets
  the same ``rgba(0,0,0,0.04)`` tint as the hover state (no white
  pill, no shadow), only the heavier weight + ink text distinguishes
  it.

- OS app icon (page_icon) switched from SVG to a Pillow-rendered
  ``datatools_icon_256.png`` so Windows / macOS taskbar+dock pick
  it up reliably (some OS shells fall back to a default icon for
  SVG favicons). Rounded-square ink ground with cream "D" centered —
  same mark as the sidebar chip + hero chip.

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

src/gui/_home.py

@@ -172,7 +172,7 @@ def _home_page() -> None:
     from src.i18n import t
 
     from pathlib import Path as _Path
-    _ICON_PATH = str(_Path(__file__).parent / "assets" / "datatools_icon.svg")
+    _ICON_PATH = str(_Path(__file__).parent / "assets" / "datatools_icon_256.png")
     st.set_page_config(
         page_title=t("home.page_title"),
         page_icon=_ICON_PATH,
@@ -182,17 +182,22 @@ def _home_page() -> None:
     render_sticky_footer()
 
     import html as _html
-    # Page header — h1 + body subtitle on the left, privacy pill on
-    # the right (mockup §page-header). Rendered as a single HTML block
-    # so the title/subtitle/pill share one flex row; ``st.title`` +
-    # ``st.caption`` + ``st.divider`` would stack vertically and lose
-    # the right-aligned pill. Bottom border replaces the explicit
-    # ``st.divider`` that used to sit below the caption.
+    # Page header — brand block (D icon + "UNALOGIX" eyebrow over
+    # "DataTools" wordmark + tagline) on the left, privacy pill on
+    # the right. Matches the sidebar brand chip scaled up for the
+    # hero. Bottom border replaces the explicit ``st.divider`` that
+    # used to sit below the caption.
     privacy_label = _html.escape(t("home.privacy_pill"))
     st.markdown(
         '<header class="dt-page-header">'
-        '<div>'
-        f'<h1>{_html.escape(t("home.title"))}</h1>'
+        '<div class="dt-page-brand">'
+        '<div class="dt-page-brand-row">'
+        '<div class="dt-page-brand-mark">D</div>'
+        '<div class="dt-page-brand-words">'
+        '<span class="dt-page-eyebrow">UNALOGIX</span>'
+        '<h1 class="dt-page-wordmark">DataTools</h1>'
+        '</div>'
+        '</div>'
         f'<p class="dt-page-subtitle">{_html.escape(t("home.caption"))}</p>'
         '</div>'
         '<span class="dt-privacy-pill">'