feat(ui): page header + files card + action bar + findings cards (mockup 2)

Closes the remaining gaps between the live home page and the
``datatools_layout_redesign2.html`` mockup. Four pieces land
together because they all consume the same new CSS scaffold:

1. Page header (§page-header)
   ``st.title`` + ``st.caption`` + ``st.divider`` collapse into one
   flex header: h1 + body subtitle on the left, ``Runs 100% locally``
   privacy pill (success-fill + lock SVG) on the right, soft border
   below. The "Runs 100% locally" phrase moved out of
   ``home.caption`` into the new ``home.privacy_pill`` i18n key
   (en + es).

2. Files card (§files-card)
   The "Imported files" list is now a single bordered card with a
   section head (count + KB total on the right, mockup §section-head).
   Each row renders a 28px accent-fill chip carrying the inline
   document SVG, a mono filename, a right-aligned mono size, and a
   compact ``✕`` button. The word-button ``Remove`` is gone —
   replaced by an icon-only tertiary button styled via a new CSS
   rule that goes transparent → danger-fill on hover (mockup
   §file-remove).

3. Action bar (§action-bar)
   Three buttons in one row: ``Run analysis`` (primary ink), a new
   disabled ``Export report`` (secondary; coming soon, tooltip), and
   ``Clear results``. New i18n key ``upload.export_report``.

4. Findings — per-file group cards (§finding-group)
   ``render_findings_panel`` rewritten end-to-end. Output is now:
     • A head row (``dt-finding-group-head``) bleeding to the card
       edges: worst-severity dot · mono filename · count pills
       enumerating non-zero severities (e.g. ``2 info`` blue,
       ``1 warning`` amber, ``1 error`` rose).
     • A flat list of finding rows sorted error → warn → info.
       Each row: tinted Material-icon chip + title (description
       with optional ``<code>`` column chip) + mono meta line
       (rows affected, samples captured) + tertiary
       ``Open <Tool> →`` action button that ``st.switch_page``s
       to the relevant tool.
   The previous tool-grouped expander stack is dropped — the new
   layout is denser and matches the mockup's single-card-per-file
   structure.

   ``_render_one_finding`` (the old per-finding helper that emitted
   markdown lines + sample tables) remains in the file but is no
   longer called from the home flow; left in place for any other
   surface that still depends on the markdown style.

   The "no issues" success state renders a green dot + mono
   filename + ``no issues`` success pill in the same card chrome,
   so empty-result files visually match the rest of the panel
   rather than getting a generic ``st.success`` callout.

CSS additions (``_DESIGN_TOKENS_CSS``):
  ``.dt-page-header / .dt-page-subtitle / .dt-privacy-pill``
  ``.dt-files-section-head / .dt-section-meta``
  ``.dt-file-row / .dt-file-icon-chip / .dt-file-name / .dt-file-size``
  ``.dt-finding-group-head / .dt-severity-dot{.warn,.info,.error,.success}``
  ``.dt-group-filename / .dt-group-counts``
  ``.dt-count-pill{.warn,.info,.error,.success}``
  ``.dt-finding-row / .dt-finding-icon{.warn,.info,.error}``
  ``.dt-finding-title / .dt-finding-meta``
  Tertiary button rule (transparent → danger-fill on hover) for
  the X button and the ``Open Tool →`` row action.

theme.py:
  Explicitly loads Material Symbols Outlined alongside Geist —
  the severity-chip ligatures (``info`` / ``warning`` / ``error``)
  need the font present even when no ``:material/`` token has been
  emitted yet on the page. Tightened ``.dt-finding-icon .dt-mui``
  selector with ``[data-testid="stMarkdownContainer"]``-scoped
  variant so the Material font wins over theme.py's base
  ``var(--font-sans) !important`` on markdown descendants.

Leading section-heading emojis stripped from i18n
(``upload.heading``) for parity with the mockup's clean ``Files``
/ ``Findings`` h2s.

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

src/i18n/packs/en.json

@@ -6,7 +6,8 @@
   "home": {
     "page_title": "DataTools — Data Cleaning Mastery",
     "title": "DataTools — Data Cleaning Mastery",
-    "caption": "A 9-tool suite for cleaning, standardizing, and validating tabular data. Runs 100% locally.",
+    "caption": "A 9-tool suite for cleaning, standardizing, and validating tabular data.",
+    "privacy_pill": "Runs 100% locally",
     "findings_badge_one": "{n} finding",
     "findings_badge_other": "{n} findings"
   },
@@ -15,7 +16,7 @@
     "coming_soon": "Coming Soon"
   },
   "upload": {
-    "heading": "📤 Import one or more files to start",
+    "heading": "Import one or more files to start",
     "intro": "Optional: scan an imported file for data quality issues and see which tools can fix each one. Skip if you already know what you need.",
     "limits": "**Up to 1.5 GB.** Formats: CSV, TSV, XLSX, XLS. Delimiters auto-detected: comma, tab, semicolon, pipe. Encodings auto-detected: UTF-8 (with/without BOM), UTF-16, cp1252, Latin-1/9, cp1250, ISO-8859-2, cp1251, KOI8-R, Mac Roman, Shift_JIS, GB18030, Big5, EUC-KR — and override on the Review page.",
     "uploader_label": "Import CSV or Excel",
@@ -31,6 +32,7 @@
     "intro_multi": "Drop files below. Each one is analyzed locally — nothing leaves this computer.",
     "uploader_label_multi": "Import CSV, TSV, or Excel files",
     "clear_results": "Clear results",
+    "export_report": "Export report",
     "empty_state": "Import one or more files to begin. Your data never leaves this computer."
   },
   "findings": {