fix: GitLab repo_id mishandled group/subgroup-owned projects

repo_id() always prefixed self.namespace onto the given repo name, so
any repo already passed as a full group/repo or group/subgroup/repo
path got mangled into namespace/group/repo and 404'd on every
settings-tab mutation. Now mirrors GitHubProvider::slug: an
already-qualified path (contains '/') is used verbatim.

src/providers/gitlab.rs

@@ -42,10 +42,13 @@ impl GitLabProvider {
 
     /// Builds GitLab's URL-encoded project identifier (`namespace%2Frepo`),
     /// since the v4 API addresses projects by encoded path rather than a
-    /// separate numeric id field this codebase tracks.
+    /// separate numeric id field this codebase tracks. `repo` may already be
+    /// a full `group/subgroup/repo` path (mirrors `GitHubProvider::slug`) —
+    /// in that case it's used verbatim instead of prefixing `self.namespace`
+    /// again, which would otherwise mangle any group- or subgroup-owned
+    /// project into a nonexistent `namespace/group/repo` path.
     fn repo_id(&self, repo: &str) -> String {
-        // Percent-encode "namespace/repo" for use in URL path segments
-        let raw = format!("{}/{}", self.namespace, repo);
+        let raw = if repo.contains('/') { repo.to_string() } else { format!("{}/{}", self.namespace, repo) };
         raw.replace('/', "%2F")
     }