fix(results-page): empty input history with zen mode (@byseif21)

[byseif21]

fix some issue in input history in zen mode test

• the results history render empty entries because the code still tried to read target words
• always shows wpm infinity for last words

[Copilot]

Pull request overview

Fixes results-page word history rendering when TestWords.words.get(i) is missing (notably in zen mode), preventing failures when building per-word history UI.

Changes:

• Default missing TestWords.words.get(i) to "" while building results word history.
• Remove redundant ?? "" at character-splitting callsite by guaranteeing word is always a string.

Comments suppressed due to low confidence (1)

frontend/src/ts/test/test-ui.ts:1332

• containsKorean check is wrong when input is undefined: input?.match(...) returns undefined, and undefined !== null becomes true, so containsKorean can be true even with no input. Use a boolean cast instead (e.g. Boolean((input ?? "").match(re)) / re.test(input ?? "")) and drop the now-unnecessary optional chain on word since it’s always a string here.

    const containsKorean =
      input?.match(
        /[\uac00-\ud7af]|[\u1100-\u11ff]|[\u3130-\u318f]|[\ua960-\ua97f]|[\ud7b0-\ud7ff]/g,
      ) !== null ||
      word?.match(