fix(table): resize grouped header columns correctly

[xonaib]

Summary

• measure the active resize header row instead of every header cell in the table head
• keep a fallback for tables without an active resize column
• add regression coverage for resizable columns under grouped headers

Fixes #78

Test plan

• pnpm --filter @openng/optimus-ui test:unit --include=src/table/table.spec.ts
• pnpm run build:lib

[xonaib]

I saw there was an older PR/comment around this issue, but that PR appears to have been closed as stale/too old, with a note that a fresh PR would be welcome if the issue is still relevant. This is a fresh pass against the current codebase and includes regression coverage for grouped headers.

Tested locally:

• pnpm --filter @openng/optimus-ui test:unit --include=src/table/table.spec.ts
• pnpm run build:lib