feat(ui): Material icons in sidebar + stats overview on home

Two pieces of the mockup 2 layout that hadn't landed yet:

1. Sidebar nav icons — emoji glyphs (🧹 ✂️ 🔍 …) swapped for
   Streamlit's ``:material/<name>:`` syntax, picking the outline
   Material Symbol that best matches each mockup SVG:

       Home               → :material/home:
       Fix Missing Values → :material/help_outline:
       Find Unusual Vals  → :material/insights:
       Clean Text         → :material/text_format:
       Standardize Fmts   → :material/format_list_bulleted:
       Find Duplicates    → :material/search:
       Quality Check      → :material/check_circle:
       Map Columns        → :material/view_column:
       Combine Files      → :material/account_tree:
       Auto Workflows     → :material/auto_awesome:
       Activate           → :material/key:
       Close              → :material/close:

   Streamlit injects the icon name as a literal ligature inside a
   first-child ``<span>`` of the nav anchor, expected to render
   through the Material Symbols font. theme.py's base rule was
   forcing Geist on every span under ``stSidebarNav``, turning the
   ligatures back into plain text labels — added a structural
   exception that targets ``[data-testid="stSidebarNavLink"] >
   span:first-child`` (and any descendant), restoring the Material
   font family, neutralizing the inherited ``ss01/cv01/cv11``
   feature settings, and sizing to 18px.

   Also stripped the leading emojis from every page title in the
   en/es i18n packs (``home.title``, ``close_page.title``,
   ``activation.title``, ``tools.*.page_title``) — the icons live
   in the sidebar now, the page H1 no longer needs to carry one.

2. Stats overview on home — new ``_render_stats_overview`` in
   _home.py emits a 4-card grid above the per-file findings panels:
   Files analyzed, Total findings, Warnings (severity ``warn`` ∪
   ``error``), Info (severity ``info``). Card layout follows the
   mockup §stats verbatim — Geist 28px / 600 / -0.03em for the
   numeric value (the "Display number" row in spec §4), tiny
   uppercase tracked label, paper-surface card with the standard
   warm border + faint shadow. The Warnings / Info cards tint the
   number with ``--warn`` / ``--info`` when the count is non-zero.

CSS for ``.dt-stats / .dt-stat / .dt-stat-label / .dt-stat-value /
.dt-stat-unit`` added to ``_DESIGN_TOKENS_CSS``; falls to a
2-column grid below 900px viewport, matching the mockup's media
query.

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

src/gui/tools_registry.py

@@ -49,7 +49,7 @@ class Tool:
 TOOLS: list[Tool] = [
     Tool(
         tool_id="04_missing_handler",
-        icon="🕳️",
+        icon=":material/help_outline:",
         name="Fix Missing Values",
         description=(
             "Detect disguised nulls, missingness analysis, and imputation strategies."
@@ -60,7 +60,7 @@ TOOLS: list[Tool] = [
     ),
     Tool(
         tool_id="06_outlier_detector",
-        icon="📊",
+        icon=":material/insights:",
         name="Find Unusual Values",
         description=(
             "Z-score, IQR, and MAD detection with domain-rule violations and "
@@ -72,7 +72,7 @@ TOOLS: list[Tool] = [
     ),
     Tool(
         tool_id="02_text_cleaner",
-        icon="✂️",
+        icon=":material/text_format:",
         name="Clean Text",
         description=(
             "Whitespace trim, multi-space collapse, Unicode normalization, "
@@ -84,7 +84,7 @@ TOOLS: list[Tool] = [
     ),
     Tool(
         tool_id="03_format_standardizer",
-        icon="📐",
+        icon=":material/format_list_bulleted:",
         name="Standardize Formats",
         description=(
             "Standardize dates, currencies, names, phone numbers, and addresses."
@@ -95,7 +95,7 @@ TOOLS: list[Tool] = [
     ),
     Tool(
         tool_id="01_deduplicator",
-        icon="🔍",
+        icon=":material/search:",
         name="Find Duplicates",
         description=(
             "Fuzzy matching, normalization, survivor selection, and "
@@ -107,7 +107,7 @@ TOOLS: list[Tool] = [
     ),
     Tool(
         tool_id="08_validator_reporter",
-        icon="✅",
+        icon=":material/check_circle:",
         name="Quality Check",
         description=(
             "Validate against rules and generate PDF/Excel quality reports."
@@ -118,7 +118,7 @@ TOOLS: list[Tool] = [
     ),
     Tool(
         tool_id="05_column_mapper",
-        icon="🗂️",
+        icon=":material/view_column:",
         name="Map Columns",
         description="Rename columns, enforce a target schema, and coerce types.",
         page_slug="5_Column_Mapper",
@@ -127,7 +127,7 @@ TOOLS: list[Tool] = [
     ),
     Tool(
         tool_id="07_multi_file_merger",
-        icon="📎",
+        icon=":material/account_tree:",
         name="Combine Files",
         description="Combine multiple CSV/Excel files with schema alignment.",
         page_slug="7_Multi_File_Merger",
@@ -136,7 +136,7 @@ TOOLS: list[Tool] = [
     ),
     Tool(
         tool_id="09_pipeline_runner",
-        icon="⚙️",
+        icon=":material/auto_awesome:",
         name="Automated Workflows",
         description=(
             "Chain tools in recommended order and pass output between steps."