feat(brand): "Letter D (sans)" app icon — favicon + sidebar chip

Implements ``Business/DataTools/app_icons.html`` §03 "Letter D (sans)"
as the canonical app mark.

- New ``src/gui/assets/datatools_icon.svg`` — 64×64 SVG, 14px corner
  radius, ink ground (#1c1917), cream "D" (#fef4ed) in
  Geist 700 / -0.04em tracking. Pure SVG so it renders sharp at
  every favicon size; font stack falls back through Geist →
  system sans where the webfont isn't installed (favicons can't load
  Google Fonts).

- ``_home.py``, ``_Activate.py``, ``99_Close.py``: page_icon now
  resolves the SVG path via ``Path(__file__).parent / "assets" /
  "datatools_icon.svg"`` instead of the broom 🧹 / 🔑 / 🛑
  emojis. Streamlit inlines it as a ``data:image/svg+xml;base64,...``
  link tag so the browser tab + OS app-icon for ``python -m src.gui``
  matches the sidebar chip.

- Sidebar ``.dt-brand-mark`` tightened to match the spec's "Letter D
  (sans)" rendering: ``font-weight: 700`` and
  ``letter-spacing: -0.04em`` (was 600 / -0.02em). The on-screen
  chip is now a scaled-up copy of the OS icon.

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

src/gui/_home.py

@@ -171,9 +171,11 @@ def _home_page() -> None:
     from src.gui.components._legacy import _run_analysis_on_upload
     from src.i18n import t
 
+    from pathlib import Path as _Path
+    _ICON_PATH = str(_Path(__file__).parent / "assets" / "datatools_icon.svg")
     st.set_page_config(
         page_title=t("home.page_title"),
-        page_icon="🧹",
+        page_icon=_ICON_PATH,
         layout="wide",
     )
     hide_streamlit_chrome()

src/gui/assets/datatools_icon.svg

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  UNALOGIX DataTools app icon. Letter "D" (sans wordmark) on ink ground —
+  matches Business/DataTools/app_icons.html §03 "Letter D (sans)".
+  Used as the Streamlit favicon (page_icon) and the sidebar brand chip.
+  Pure SVG so it renders crisp at every favicon size; cream "D" on
+  ``--ink`` background, font-family stack falls back to system sans
+  where Geist isn't installed.
+-->
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" width="64" height="64">
+  <rect width="64" height="64" rx="14" fill="#1c1917"/>
+  <text x="32" y="48"
+        text-anchor="middle"
+        font-family="Geist, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif"
+        font-weight="700"
+        font-size="44"
+        letter-spacing="-1.76"
+        fill="#fef4ed">D</text>
+</svg>

src/gui/components/_legacy.py

@@ -201,6 +201,10 @@ body, .stApp {
     height: 100%;
     flex: 1;
 }
+/* "Letter D (sans)" wordmark per Business/DataTools/app_icons.html
+   §03: 28px ink-filled rounded square, cream "D" in Geist 700 with
+   -0.04em tracking. Same shape used for the favicon SVG so the chip
+   in the sidebar reads as a scaled-up copy of the OS app icon. */
 .dt-brand-mark {
     width: 28px;
     height: 28px;
@@ -211,9 +215,9 @@ body, .stApp {
     justify-content: center;
     color: var(--accent-fill);
     font-family: var(--font-sans);
-    font-weight: 600;
+    font-weight: 700;
     font-size: 16px;
-    letter-spacing: -0.02em;
+    letter-spacing: -0.04em;
     line-height: 1;
     flex-shrink: 0;
 }

src/gui/pages/99_Close.py

@@ -20,9 +20,11 @@ if str(_project_root) not in sys.path:
 from src.gui.components import hide_streamlit_chrome, shutdown_app
 from src.i18n import t
 
+from pathlib import Path as _Path
+_ICON_PATH = str(_Path(__file__).parent.parent / "assets" / "datatools_icon.svg")
 st.set_page_config(
     page_title=t("close_page.page_title"),
-    page_icon="🛑",
+    page_icon=_ICON_PATH,
     layout="wide",
 )
 hide_streamlit_chrome()

src/gui/pages/_Activate.py

@@ -26,9 +26,11 @@ from src.gui.components import (
 )
 from src.i18n import t
 
+from pathlib import Path as _Path
+_ICON_PATH = str(_Path(__file__).parent.parent / "assets" / "datatools_icon.svg")
 st.set_page_config(
     page_title=t("activation.page_title"),
-    page_icon="🔑",
+    page_icon=_ICON_PATH,
     layout="wide",
 )