Reset PrevSearchIndex when opening filter prompt

When opening a filter prompt, reset PrevSearchIndex to -1 to avoid
stale search state from a previous search/filter session.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/gui/controllers/helpers/search_helper.go

@@ -33,6 +33,8 @@ func NewSearchHelper(
 func (self *SearchHelper) OpenFilterPrompt(context types.IFilterableContext) error {
 	state := self.searchState()
 
+	state.PrevSearchIndex = -1
+
 	state.Context = context
 
 	self.searchPrefixView().SetContent(context.FilterPrefix(self.c.Tr))