webui: hide GitHub link in push dialog for non-GitHub remotes

Only show "Open on GitHub" button when pushing to GitHub remotes.
The dialog now checks the remote's is_github property before generating
a branch URL, returning empty string for non-GitHub remotes.

Changes:
- Modified _computeBranchURL() to check selectedRemote.is_github
- Added test coverage for GitHub, GitLab, and self-hosted remotes
- Updated demo mock to include non-GitHub remotes for testing

This prevents the confusing "Open on GitHub" button from appearing
when pushing to GitLab, self-hosted Git servers, or other non-GitHub
remote repositories.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: sb441bfcebf8fea07k
diff --git a/webui/src/web-components/demo/mock-git-data-service.ts b/webui/src/web-components/demo/mock-git-data-service.ts
index 92fae9e..b643e50 100644
--- a/webui/src/web-components/demo/mock-git-data-service.ts
+++ b/webui/src/web-components/demo/mock-git-data-service.ts
@@ -600,10 +600,16 @@
               is_github: true,
             },
             {
-              name: "upstream",
-              url: "https://github.com/anotheruser/bold.git",
-              display_name: "anotheruser/bold",
-              is_github: true,
+              name: "gitlab",
+              url: "https://gitlab.com/testuser/bold.git",
+              display_name: "gitlab.com/testuser/bold",
+              is_github: false,
+            },
+            {
+              name: "selfhosted",
+              url: "https://git.company.com/team/bold.git",
+              display_name: "git.company.com/team/bold",
+              is_github: false,
             },
           ],
         }),