Fix Auto Parse TypeError from invalid OpenCV.js column data

[Copilot]

Auto Parse crashes with TypeError: Cannot create property '3918' on number '1' when OpenCV.js returns malformed column data during text line detection.

Changes

Added validation helper

• isValidColumn(col) validates array structure and numeric types before destructuring

Defensive column merging

• Validate columns[0] before accessing elements
• Check each column element in merge loop, skip invalid entries
• Use explicit assignments instead of destructuring reassignment
• Fall back to full-width column [[0, binary.cols]] on invalid data

Safe reduce operation

• Validate each column element before arithmetic
• Skip malformed entries instead of crashing

Example

// Before: crashes on invalid data
let [curStart, curEnd] = columns[0]  // TypeError if columns[0] is not an array
for (const [s, e] of columns.slice(1)) { ... }

// After: validates and handles gracefully
if (!isValidColumn(columns[0])) {
  console.warn("Invalid column format detected, using default column")
  columns = [[0, binary.cols]]
} else {
  let curStart = columns[0][0]
  let curEnd = columns[0][1]
  for (const col of columns.slice(1)) {
    if (!isValidColumn(col)) continue
    const [s, e] = col
    ...
  }
}

Invalid data now logs warnings and degrades to single-column detection rather than crashing.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Auto Parse is Failing</issue_title>
<issue_description>On my localhost.

I am trying it on the first page with a project created from https://tpen-project-examples.habesoftware.app/transcription-project/manifest.json. I get an error toast and see this in the web console. This is consistent.

Auto Parse failed: TypeError: Cannot create property '3918' on number '1'
    at detectColumns (detect-lines.js:58:28)
    at detectTextLinesCombined (detect-lines.js:155:19)
    at async HTMLButtonElement.<anonymous> (line-parser.js:307:25)
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Auto Parse is Failing #357

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