Fix unsafe URL launch in AttachesRenderer

[Copilot]

AttachesRenderer was calling Uri.parse (throws on malformed input) and passing the result directly to launchUrl with no scheme validation, allowing arbitrary URI schemes from document data.

Changes

• attaches_renderer.dart: Replace unsafe one-liner with the safe async pattern already established in EmbedRenderer and LinkToolRenderer:
  • Uri.tryParse instead of Uri.parse — returns null on malformed input rather than throwing
  • Scheme allowlist (http/https only) — blocks javascript:, file:, etc.
  • canLaunchUrl pre-check before invoking launchUrl

// Before
() => launchUrl(Uri.parse(block.url), mode: LaunchMode.externalApplication)

// After
() async {
  final uri = Uri.tryParse(block.url);
  if (uri == null) return;
  final scheme = uri.scheme.toLowerCase();
  if (scheme != 'http' && scheme != 'https') return;
  if (await canLaunchUrl(uri)) {
    await launchUrl(uri, mode: LaunchMode.externalApplication);
  }
}

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