feat(diff): Add search and extension filter to diff sidebar (#37068)

Adds a search box and a file-extension filter to the pull request diff
sidebar, so reviewers can narrow a large diff down to the files they
care about.

Both filters apply to the file tree and to the diff itself. The
extension menu follows GitHub: extensions sorted alphabetically,
dotfiles and extension-less files in their own buckets, and the
selection kept in the same `file-filters[]` query parameter, so a
filtered view is shareable and survives a reload.

The menu can list every extension in a diff, so `createTippy` gains an
opt-in `limitSizeToViewport` option that caps a popup to the space left
in the viewport and scrolls its content. Popups that do not ask for it
are unchanged.

Closes https://github.com/go-gitea/gitea/issues/27256
Signed-off-by: silverwind <me@silverwind.io>
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: Copilot <copilot@github.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/utils.test.ts

@@ -17,6 +17,9 @@ test('basename', () => {
 });
 
 test('extname', () => {
+  expect(extname('.gitignore')).toEqual('');
+  expect(extname('/path/to/.gitignore')).toEqual('');
+  expect(extname('/path/to/.eslintrc.json')).toEqual('.json');
   expect(extname('/path/to/file.js')).toEqual('.js');
   expect(extname('/path/')).toEqual('');
   expect(extname('/path')).toEqual('');