fix: remove broken stale request check and handle network errors in autocomplete

[Sherley-Sonali]

⚠️ Acceptance Requirements

• ✅ Linked to Auto-completion is often very slow #6160
• ✅ Assigned to @Sherley-Sonali

What

• Removed the broken time-based stale request check in _getSuggestions.
  start.difference(DateTime.now()).inSeconds always returns a negative value,
  so the condition > 5 never triggered. This was dead code.
• Stale request ordering is already handled correctly by AutocompleteManager
  in openfoodfacts-dart, this confirms the "to be confirmed" from the issue discussion.
• Fixed silent catch block: on network failure, the loading spinner now always
  hides and empty results are returned cleanly instead of leaving the user
  with a stuck spinner.

Screenshot or video

No visual change — this is a logic fix. Spinner now correctly disappears
on network failure instead of staying indefinitely.

Fixes bug(s)

• Fixes: Auto-completion is often very slow #6160

Part of

• Server-side issues (tracker) #1462

[monsieurtanuki]

Hi @Sherley-Sonali!
The first step would be to format your code, cf. dart format .