fix(i18n): comprehensive translation audit and fixes across all 14 languages

- Fix critical bug: Photos and Files pages had German text hardcoded in JSX,
  now use t() keys visible correctly in all languages
- Add 16 new translation keys (photos/files UI, login validation, common errors,
  rate limit message) across all 14 language files
- Add missing keys in packing, memories, and budget sections for br, de, it, es,
  fr, nl, pl, cs, hu, ru, zh, zh-TW, ar
- Add 152+ missing keys for zh-TW (entire sections were absent)
- Change Vacay addon name to 'Férias' in pt-BR only
- Add client-side HTTP 429 interceptor that shows translated rate limit message
- Replace hardcoded English fallbacks in TripPlannerPage, DayPlanSidebar,
  DisplaySettingsTab, MapSettingsTab, AccountTab, and TodoListPanel with t()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/components/Files/FileManager.tsx

@@ -778,7 +778,7 @@ export default function FileManager({ files = [], onUpload, onDelete, onUpdate,
               title={previewFile.original_name}
             >
               <p style={{ padding: 24, textAlign: 'center', color: 'var(--text-muted)' }}>
-                <button onClick={async () => { const u = await getAuthUrl(previewFile.url, 'download'); window.open(u, '_blank', 'noopener noreferrer') }} style={{ color: 'var(--text-primary)', textDecoration: 'underline', background: 'none', border: 'none', cursor: 'pointer', font: 'inherit' }}>PDF herunterladen</button>
+                <button onClick={async () => { const u = await getAuthUrl(previewFile.url, 'download'); window.open(u, '_blank', 'noopener noreferrer') }} style={{ color: 'var(--text-primary)', textDecoration: 'underline', background: 'none', border: 'none', cursor: 'pointer', font: 'inherit' }}>{t('files.downloadPdf')}</button>
               </p>
             </object>
           </div>