Fix selection ranges that start with whitespace

[happykawayigt]

Summary

Fixes #64 by deriving the shared media fragment from spans that contribute actual non-whitespace selection text.

Changes

• intersect the selection range with each timed span before choosing boundaries
• ignore spans where the intersection contains only whitespace
• remove the leading-space index adjustment that could skip the first selected word
• regenerate the ESM build and add regression coverage

Validation

• node scripts/build-mjs.js
• full Jest suite: 3 suites passed, 75 tests passed
• direct jsdom reproduction: selection " two thr" resolves to 2.00,3.50 instead of 3.00,3.50
• git diff --check