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/translate/page.tsx

@@ -20,6 +20,7 @@ import { useNotification } from '@/components/ui/notification';
 import { useI18n } from '@/lib/i18n';
 import { API_BASE } from '@/lib/config';
 import { cn } from '@/lib/utils';
+import { useUser } from '../useUser';
 
 /* ── helpers ─────────────────────────────────────────────────────── */
 const FILE_ICONS: Record<string, React.ElementType> = {
@@ -74,6 +75,8 @@ export default function TranslatePage() {
   const submit = useTranslationSubmit();
   const { error: showError } = useNotification();
   const { t } = useI18n();
+  const { data: currentUser } = useUser();
+  const isPaid = !!currentUser?.tier && currentUser.tier !== 'free';
   const lastErrorRef = useRef<string | null>(null);
   const replaceInputRef = useRef<HTMLInputElement>(null);
   const dropzoneInputRef = useRef<HTMLInputElement>(null);
@@ -742,8 +745,8 @@ export default function TranslatePage() {
           </div>
         </div>
 
-        {/* ── MEMENTO PROMO BANNER ──────────────────────────────── */}
-        {(showUpload || showConfiguring || showFailed) && (
+        {/* ── MEMENTO PROMO BANNER — hidden for paying users ────── */}
+        {!isPaid && (showUpload || showConfiguring || showFailed) && (
           <a
             href={t('memento.url', { defaultValue: 'https://memento-note.com/' })}
             target="_blank"