feat(repo): add quick repository switcher to repo header (#38188)

Add a GitHub-style quick repo switcher: a caret next to the owner/repo
breadcrumb
opens a dropdown that lists and searches the current owner's
repositories and
navigates to the selected one. The current repository is marked with a
check, and
private/fork repos show an icon.

Also, fix various bugs in fomtantic dropdown remote query

## Screenshots

<img width="505" height="198" alt="image"
src="https://github.com/user-attachments/assets/9f673d1b-fe60-41f0-b9e2-b00dc43720b5"
/>

Fixes #38187

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/css/modules/dropdown.css

@@ -939,6 +939,10 @@ the "!important" is necessary to override Fomantic UI menu item styles, meanwhil
   margin: 0;
 }
 
+.ui.dropdown .menu.items-width-limit .item {
+  max-width: min(500px, calc(100vw - 2em));
+}
+
 .ui.dropdown.ellipsis-text-items {
   /* reset y padding and use the line-height below instead, to avoid the "overflow: hidden" clips the larger image in the "text" element */
   padding-top: 0;