fix: add pluralized translations for stream counts and batch results

[Cashman-Exchange]

Overview

This PR adds pluralized translation keys for stream counts and batch-result messages. It covers zero, one, and many forms for counts, successes, failures, and skipped records, and replaces hardcoded English count strings with locale-aware t() lookups in the affected components.

Related Issue

Changes

🌐 Pluralized Stream Counts

• [MODIFY] src/components/CreateStreamModal.tsx
  • Use pluralized translation keys for stream counts (zero, one, many).
• [MODIFY] src/components/RecentStreams.tsx
  • Replace hardcoded stream-count strings with t('streams.count', { count }).
• [MODIFY] src/components/StreamTimeline.tsx
  • Use pluralized keys for stream count and batch result summary messages.

🌐 Pluralized Batch Results

• [MODIFY] src/components/csv-upload/PreviewValidateStep.tsx
  • Use pluralized keys for batch successes, failures, and skipped records.
• [ADD] Pluralized translation keys for supported locales, with {{count}} placeholders for stream counts and batch results.
• [ADD] Locale coverage tests:
  • src/components/csv-upload/__tests__/PreviewValidateStep.test.tsx — asserts zero/one/many forms for batch result messages.
  • src/components/__tests__/RecentStreams.test.tsx — asserts pluralized stream counts.
  • src/components/__tests__/StreamTimeline.duration.test.tsx — asserts pluralized stream count/duration messages.

Verification Results

npm test -- src/components/csv-upload/__tests__/PreviewValidateStep.test.tsx src/components/__tests__/RecentStreams.test.tsx src/components/__tests__/StreamTimeline.duration.test.tsx
✅ All affected component tests pass

Locale checks:
✅ zero / one / many forms asserted for every affected message
✅ Supported locale keys have matching placeholders
✅ No hardcoded English count strings remain in touched components

Acceptance Criteria	Status
Zero, one, and many forms are covered	✅ Added zero/one/many keys for stream counts and batch successes, failures, and skipped records
Supported locales have matching keys and placeholders	✅ Locale tests verify key/placeholder parity for every affected message
No count is interpolated into a hardcoded English string	✅ Touched components use t() pluralized keys only

Closes #1570

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