fix: allow zero viewport height with string row heights

[fastfrwrd]

Ports the fix from https://github.com/grafana/react-data-grid-patches/pull/2 into this fork. String row heights now accept a zero viewport height instead of throwing, using gridHeight ?? 0 for the total row height.

Unlike the source repository, this branch initializes dimensions to 1. The crash still occurs when layout measures a zero-height viewport, such as a hidden grid. The regression test mounts an auto-height grid in a zero-height viewport, then resizes it and verifies that rows still render.

Based on #7 (chore/sync-comcast-upstream), which introduces the required string row-height support; main does not have it yet. Merge this into that branch before #7, or retarget to main after #7 merges. No generated distribution patch is needed in this source repository.

Validation:

• New regression fails with the original error in both Chromium and Firefox before the fix.
• All 28 row-height tests pass across Chromium and Firefox with the fix (Node 26.1.0).
• Typecheck, ESLint, and formatting pass. Library build passes with ecij sourcemap warnings.
• Full browser/Node suite: 362 passed, 2 skipped, 3 Chromium sorting failures. The same three failures reproduce with the unchanged base implementation: multi column sort, sortPriority defined using both contexts, and sortPriority defined using both contexts and renderers.