fix(translate): apply Excel chart text setters (were silently dropped)

Chart <a:t> elements (title, axis labels, series names) were collected and
sent to the LLM, but the apply loop never invoked their setters. Reason:
the apply loop iterated only text_elements[:sheet_name_offset], which
excluded the chart-text positions (sheet names were inserted in the
middle, pushing chart texts past sheet_name_offset). The LLM correctly
returned French for "Revenue by Product" / "Quantity Trend" / "Order #"
but the result was thrown away.

Fix: include the chart-text slice [sheet_name_offset + N .. total_texts]
in the apply loop. Setters for sheet names are still None, so they are
naturally skipped.

Add test_excel_chart_text_applied.py (2 end-to-end tests) using a fixed
provider that pre-translates every known chart text; the test asserts
all chart <a:t> values in the output .xlsx come from the FR table, not
the original English source.

translators/excel_translator.py

@@ -264,11 +264,20 @@ class ExcelTranslator:
                                 }
                             )
 
-                # Apply cell translations
+                # Apply cell + chart translations. The structure of
+                # text_elements is:
+                #   [0 .. sheet_name_offset)                 -> cell/header/footer setters
+                #   [sheet_name_offset .. +N)                 -> sheet names (setter=None)
+                #   [sheet_name_offset + N .. total_texts)    -> chart setters
+                # The old code only iterated the first slice, silently skipping
+                # every chart setter. We now concatenate the two non-None slices.
+                _n_sheet_names = len(sheet_names_to_translate)
+                _chart_slice = text_elements[sheet_name_offset + _n_sheet_names:]
+                _chart_trans_slice = translated_texts[sheet_name_offset + _n_sheet_names:]
                 for i, ((original_text, setter), translated) in enumerate(
                     zip(
-                        text_elements[:sheet_name_offset],
-                        translated_texts[:sheet_name_offset],
+                        text_elements[:sheet_name_offset] + _chart_slice,
+                        translated_texts[:sheet_name_offset] + _chart_trans_slice,
                     )
                 ):
                     if translated is not None and setter is not None: