feat(home,sidebar): brand block + collapsible findings + many polish tweaks

Batch of UX tweaks the user asked for in quick succession:

- Sidebar brand block (mockup §brand) — 28px ink chip with a "D"
  wordmark plus the "DataTools" text — injected into
  ``stSidebarHeader`` by a small JS bundled into the iframe-mounted
  script that already runs from ``hide_streamlit_chrome``. The
  Streamlit ``stLogoSpacer`` is hidden when the brand block is
  present so it sits flush at the top of the sidebar.

- Findings cards are now collapsible. Each file's card head carries
  ``data-dt-collapsed="true"`` on first render; clicking the head
  flips the attribute via the new ``_WIRE_COLLAPSIBLE_FINDINGS_JS``
  (MutationObserver re-wires after reruns). A CSS rule
  ``[stElementContainer]:has(.dt-finding-group-head[data-dt-collapsed
  ="true"]) ~ *`` hides every later sibling of the head's element
  container — covers both ``stLayoutWrapper`` (the columns rows in
  this Streamlit release) and ``stElementContainer`` so the rule
  survives future Streamlit layout renames. A chevron icon
  (``chevron_right``) rotates 90° when expanded. The head itself
  gets ``cursor: pointer`` + an accent-fill hover.

- Tool-link buttons in finding rows dropped the leading ``Open`` —
  now read ``Clean Text →``, ``Standardize Formats →`` etc.

- Finding-row column order: action is now LEFT of the description,
  matching user feedback (``[icon] [Tool →] [description + meta]``).

- Head padding bumped to ``16px 22px`` so the filename has visible
  breathing room from the card's left edge (previously the mono
  filename felt like it was bleeding into the rounded corner).

- Head margin-bottom bumped to 1.5rem for breathing room before the
  first finding row when expanded; collapsed state tucks the head
  flush against the card bottom with full ``--r-lg`` corner radius
  and no visible bottom border.

- Files card row layout: ``✕`` button moved to the LEFT of the
  filename (``[✕] [chip + filename] [size]``).

- Sidebar nav rows tightened: link padding 7px → 4px, line-height
  1.25, 1px margin-bottom per li, section-header padding-top reduced.
  Plus a new ``--gap: 0.25rem`` rule for vertical blocks inside
  bordered containers so the Files card and findings card body have
  denser inter-row spacing.

- Sidebar Language selector restyled: widget labels render as the
  spec's "Eyebrow" row (11.5px / 500 / 0.08em uppercase, tertiary
  ink), selectbox combobox gets a paper surface + soft border that
  matches the rest of the sidebar chrome.

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

src/gui/_home.py

@@ -279,7 +279,16 @@ def _home_page() -> None:
             digest = hashlib.sha1(
                 name.encode("utf-8"), usedforsecurity=False,
             ).hexdigest()[:10]
-            col_name, col_size, col_x = st.columns([8, 1.6, 0.55])
+            # X button on the LEFT of the row per UX feedback —
+            # ``✕ | filename + chip | size``.
+            col_x, col_name, col_size = st.columns([0.55, 8, 1.6])
+            if col_x.button(
+                "✕",
+                key=f"_home_remove_{digest}",
+                help=f"Remove {name}",
+                type="tertiary",
+            ):
+                to_remove = name
             col_name.markdown(
                 '<div class="dt-file-row">'
                 f'<span class="dt-file-icon-chip">{_DOC_SVG}</span>'
@@ -294,13 +303,6 @@ def _home_page() -> None:
                 '</span></div>',
                 unsafe_allow_html=True,
             )
-            if col_x.button(
-                "✕",
-                key=f"_home_remove_{digest}",
-                help=f"Remove {name}",
-                type="tertiary",
-            ):
-                to_remove = name
         # In-card "Add more files" — clicks the (off-screen)
         # ``stFileUploaderDropzoneInput`` so the OS file picker opens.
         # Inline ``onclick`` would be cleanest but Streamlit's HTML