fix(translate): chart labels, sheet-refs, sheet-name offset, paid-user Memento

Multiple translation bugs in Word/Excel/PPTX that caused chart elements
to be left untranslated or charts to render as empty series.

Backend
-------
* providers (deepseek/openai/minimax): tighten system prompt so the LLM
  actually translates chart titles/axis labels/legend/category labels,
  month abbreviations, and clarifies what counts as a 'real' proper noun
  (people/place/company/product names) vs. technical labels. Old rule
  'keep proper nouns unchanged' was being read too broadly by the model
  and caused chart text to be skipped.
* excel_translator.py:
  - Sheet reference rewrite: when a sheet is renamed, the chart XML's
    c:f refs (e.g. 'Sales 2024'!$D$2:$D$61) are now rewritten to the
    new name with proper apostrophe escaping (Chiffre d'affaires ->
    'Chiffre d''affaires') and auto-quoting when the new name contains
    spaces or special chars. Without this, the chart points at a sheet
    that no longer exists and renders 0/empty series.
  - Sheet name offset bug: sheet_name_offset was computed after chart
    text was appended to text_elements, causing sheet names to receive
    chart text translations. Now captured BEFORE sheet names are added.
* New tests:
  - test_excel_chart_sheet_refs.py (10 unit tests, synthetic inputs)
  - test_chart_translation_prompt.py (3 contract tests on the prompt)

Frontend
--------
* DashboardSidebar / translate/page: hide the Memento promo section
  for paying users (tier != 'free').
* constants.ts: temporarily comment out the 'CLES API' nav item.
  Update constants.test.ts to match the new state.

All fixes are generic - no file-specific hardcoding, edge cases covered
(empty mapping, missing bang, apostrophe escaping, partial renames,
multi-series).

services/providers/deepseek_provider.py

@@ -31,8 +31,11 @@ Rules:
 - Translate ONLY the text, do not add explanations or notes
 - Preserve the original formatting, line breaks, and structure
 - Maintain the original tone and style
-- For technical terms, use the standard translation in the target language
-- If the text contains proper nouns or brand names, keep them unchanged unless there's a well-known translation"""
+- Translate technical terms, jargon, and labels using the standard target-language equivalent
+- Chart elements (titles, axis labels, legend entries, category labels, series names) MUST always be translated, even when they look like a title or a proper noun
+- Translate month and weekday abbreviations (Jan → janvier, Mon → lundi) to the target language
+- Keep ONLY real proper nouns unchanged: people's names, place names, company names (Google, Microsoft), and product names (GitHub, HuggingFace)
+- Do not invent content; if a term is an acronym with no translation (API, URL, HTTP, JSON), keep it as-is"""
 
 
 def _get_language_name(code: str) -> str:

services/providers/minimax_provider.py

@@ -31,8 +31,11 @@ Rules:
 - Translate ONLY the text, do not add explanations or notes
 - Preserve the original formatting, line breaks, and structure
 - Maintain the original tone and style
-- For technical terms, use the standard translation in the target language
-- If the text contains proper nouns or brand names, keep them unchanged unless there's a well-known translation"""
+- Translate technical terms, jargon, and labels using the standard target-language equivalent
+- Chart elements (titles, axis labels, legend entries, category labels, series names) MUST always be translated, even when they look like a title or a proper noun
+- Translate month and weekday abbreviations (Jan → janvier, Mon → lundi) to the target language
+- Keep ONLY real proper nouns unchanged: people's names, place names, company names (Google, Microsoft), and product names (GitHub, HuggingFace)
+- Do not invent content; if a term is an acronym with no translation (API, URL, HTTP, JSON), keep it as-is"""
 
 
 def _get_language_name(code: str) -> str:

services/providers/openai_provider.py

@@ -101,8 +101,11 @@ Rules:
 - Translate ONLY the text, do not add explanations or notes
 - Preserve the original formatting, line breaks, and structure
 - Maintain the original tone and style
-- For technical terms, use the standard translation in the target language
-- If the text contains proper nouns or brand names, keep them unchanged unless there's a well-known translation"""
+- Translate technical terms, jargon, and labels using the standard target-language equivalent
+- Chart elements (titles, axis labels, legend entries, category labels, series names) MUST always be translated, even when they look like a title or a proper noun
+- Translate month and weekday abbreviations (Jan → janvier, Mon → lundi) to the target language
+- Keep ONLY real proper nouns unchanged: people's names, place names, company names (Google, Microsoft), and product names (GitHub, HuggingFace)
+- Do not invent content; if a term is an acronym with no translation (API, URL, HTTP, JSON), keep it as-is"""
 
 
 def _build_system_prompt(