Inconsistent error handling between document and search notebook presets

[seanthimons]

Source: PR #156 review (round 1, item 4)

Problem

Document notebook preset handlers use simple error handling:

}, error = function(e) {
  sprintf("Error: %s", e$message)
})

Search notebook uses the more robust pattern:

}, error = function(e) {
  if (inherits(e, "api_error")) {
    show_error_toast(e$message, e$details, e$severity)
  } else {
    err <- classify_api_error(e, "OpenRouter")
    show_error_toast(err$message, err$details, err$severity)
  }
  "Sorry, I encountered an error..."
})

Suggested Fix

Apply the search notebook error pattern to document notebook preset handlers for consistent UX. Users get the same quality of error feedback regardless of which notebook type they're using.