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).

frontend/src/app/dashboard/constants.ts

@@ -1,4 +1,4 @@
-import { FileText, Key, BookText, User, type LucideIcon } from 'lucide-react';
+import { FileText, BookText, User, type LucideIcon } from 'lucide-react';
 
 export interface NavItem {
   labelKey: string;
@@ -11,5 +11,6 @@ export const baseNavItems: NavItem[] = [
   { labelKey: 'dashboard.nav.translate', href: '/dashboard/translate', icon: FileText },
   { labelKey: 'dashboard.nav.profile', href: '/dashboard/profile', icon: User },
   { labelKey: 'dashboard.nav.glossaries', href: '/dashboard/glossaries', icon: BookText, proOnly: true },
-  { labelKey: 'dashboard.nav.apiKeys', href: '/dashboard/api-keys', icon: Key, proOnly: true },
+  // API Keys nav item temporarily removed per request — uncomment to restore.
+  // { labelKey: 'dashboard.nav.apiKeys', href: '/dashboard/api-keys', icon: Key, proOnly: true },
 ];