Fix counting and filtering on the dashboard page for issues (#26657)

This PR has multiple parts, and I didn't split them because
it's not easy to test them separately since they are all about the
dashboard page for issues.

1. Support counting issues via indexer to fix #26361
2. Fix repo selection so it also fixes #26653
3. Keep keywords in filter links.

The first two are regressions of #26012.

After:

https://github.com/go-gitea/gitea/assets/9418365/71dfea7e-d9e2-42b6-851a-cc081435c946

Thanks to @CaiCandong  for helping with some tests.

modules/indexer/issues/internal/model.go

@@ -109,6 +109,19 @@ type SearchOptions struct {
 	SortBy SortBy // sort by field
 }
 
+// Copy returns a copy of the options.
+// Be careful, it's not a deep copy, so `SearchOptions.RepoIDs = {...}` is OK while `SearchOptions.RepoIDs[0] = ...` is not.
+func (o *SearchOptions) Copy(edit ...func(options *SearchOptions)) *SearchOptions {
+	if o == nil {
+		return nil
+	}
+	v := *o
+	for _, e := range edit {
+		e(&v)
+	}
+	return &v
+}
+
 type SortBy string
 
 const (