fix(cli): preserve Language header when pushing translations

[ceolinwill]

Summary

Fixes PO loader replacing target Language header with source language when pushing translations.

Changes

• Preserve headers from the target (pullInput) section when compiling updated PO sections in packages/cli/src/cli/loaders/po/index.ts.
• Add test to reproduce and validate the bug: should preserve existing Language header when pushing translations with metadata in first section in packages/cli/src/cli/loaders/po/index.spec.ts.

Testing

Business logic tests added:

• Validate that pushing translations preserves an existing Language header when metadata and the first entry are in the same section (exact test: should preserve existing Language header when pushing translations with metadata in first section).
• Validate that other PO loader behaviors (comment order, multiline folding, context handling) remain unchanged by the fix (covered by existing tests in packages/cli/src/cli/loaders/po/index.spec.ts).
• All tests pass locally

Visuals

Required for UI/UX changes:

• Before/after screenshots attached
• Video demo for interactions (< 30s)

Checklist

• Changeset added (if version bump needed)
• Tests cover business logic (not just happy path)
• No breaking changes (or documented below)

Closes #1661