feat(gui): hidden-char-aware preview tables in Text Cleaner

The Text Cleaner had two st.dataframe previews — the initial upload
preview ("Preview: filename") and the post-clean "Cleaned preview"
table — that both rendered cells with the same browser-collapses-
whitespace, hides-invisibles problem the analyzer findings panel had
before commit 1049c03.

components.render_hidden_aware_preview(df, n_rows, caption) renders a
DataFrame as an HTML table where:
  - every cell uses visualize_hidden_html(mark_outer_whitespace=True),
    so leading/trailing ASCII spaces appear as per-character "·" badges
  - white-space: pre-wrap on every cell preserves internal multi-space
    runs and embedded newlines visually
  - headers route through the same visualizer so dirty column names
    (NBSP padding, ZWSP, smart quotes) show their badges too
  - NaN cells render as a faint "NaN" placeholder
  - rows are sticky-headed and scrollable inside a 26rem capped
    container so a 10-row preview doesn't push the rest of the UI off
    screen

2_Text_Cleaner.py wires it into both previews:
  - The upload preview gains its own "Show hidden characters in preview"
    toggle (default on).
  - The cleaned preview reuses the existing show_hidden toggle that
    already governs the Examples changes table, so one switch controls
    the whole results section.

Either toggle off falls back to the original st.dataframe view.

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

src/gui/pages/2_Text_Cleaner.py

@@ -14,7 +14,11 @@ _project_root = Path(__file__).resolve().parent.parent.parent.parent
 if str(_project_root) not in sys.path:
     sys.path.insert(0, str(_project_root))
 
-from src.gui.components import hide_streamlit_chrome, pickup_or_upload
+from src.gui.components import (
+    hide_streamlit_chrome,
+    pickup_or_upload,
+    render_hidden_aware_preview,
+)
 from src.core.text_clean import (
     PRESETS,
     CleanOptions,
@@ -81,7 +85,16 @@ except Exception as e:
 
 st.subheader(f"Preview: {uploaded.name}")
 st.caption(f"{len(df)} rows, {len(df.columns)} columns")
-st.dataframe(df.head(10), use_container_width=True)
+preview_show_hidden = st.toggle(
+    "Show hidden characters in preview",
+    value=True,
+    help="Highlights NBSP, zero-width chars, smart quotes, and leading/trailing whitespace.",
+    key="textclean_preview_show_hidden",
+)
+if preview_show_hidden:
+    render_hidden_aware_preview(df, n_rows=10)
+else:
+    st.dataframe(df.head(10), use_container_width=True)
 
 st.divider()
 
@@ -257,7 +270,12 @@ if result.cells_changed:
         st.dataframe(examples, use_container_width=True, hide_index=True)
 
 st.markdown("**Cleaned preview (first 10 rows)**")
-st.dataframe(result.cleaned_df.head(10), use_container_width=True)
+# Reuse the same toggle the Examples table uses so the user controls both
+# the changes audit and the cleaned preview with one switch.
+if show_hidden:
+    render_hidden_aware_preview(result.cleaned_df, n_rows=10)
+else:
+    st.dataframe(result.cleaned_df.head(10), use_container_width=True)
 
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