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/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;
 }