Fix Show() to correctly distinguish nil vs empty enabled providers

The Show() method was using len(enabledProviders) == 0 to check for
the "all enabled" case, but this matches both nil and empty slices.
According to the Settings design:
- nil means "all enabled" (default/unset)
- empty slice means "all disabled"

Changed to enabledProviders == nil to correctly distinguish these cases.
This prevents the dialog from re-enabling all providers when the user
has explicitly disabled everything.

Fixes the last remaining issue identified in PR review.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: mzhaom

bramble/app/reposettingsdialog.go

@@ -108,8 +108,8 @@ func (d *RepoSettingsDialog) Show(repoName string, cfg RepoSettings, currentThem
 	// unchecked and untoggable in the UI.
 	d.providerStatuses = providerStatuses
 	d.enabledProviders = make(map[string]bool, len(agent.AllProviders))
-	if len(enabledProviders) == 0 {
-		// nil/empty = all installed providers enabled
+	if enabledProviders == nil {
+		// nil = all installed providers enabled (default/unset)
 		for _, s := range providerStatuses {
 			if s.Installed {
 				d.enabledProviders[s.Provider] = true