webui: remove unused diff2html-based diff view

Remove the old diff view implementation that used diff2html library
in favor of the Monaco-based diff2 view. This cleanup removes:

- sketch-diff-view.ts component that used diff2html library
- diff2html static CSS files (diff2html.min.css, diff2.css)
- diff2html npm package dependency from package.json
- Old diff view mode references throughout the codebase
- Demo files for the old diff view component

Changes include:

1. Removed Files:
   - webui/src/web-components/sketch-diff-view.ts
   - webui/src/diff2html.min.css
   - webui/src/diff2.css
   - webui/src/web-components/demo/sketch-diff-view.demo.html

2. Updated Components:
   - sketch-app-shell.ts: Remove old diff view import, ViewMode type,
     CSS selectors, and view switching logic
   - sketch-view-mode-select.ts: Update type definitions to remove 'diff' mode
   - sketch-terminal.ts: Fix view mode type and correct CSS loading comments

3. Package Management:
   - Removed diff2html 3.4.51 dependency from package.json
   - Updated package-lock.json to reflect removed dependency

4. Demo Cleanup:
   - Removed reference to old diff view demo from index.html
   - Updated timeline demo to remove diff2html from dependencies list

The Monaco-based diff2 view (sketch-diff2-view.ts) remains fully
functional and is now the only diff view implementation. All file
picker, range picker, and empty view components continue to work
with the new diff view.

Testing confirms the diff functionality works correctly after cleanup.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s22dd1dc722d02125k
diff --git a/webui/src/web-components/demo/index.html b/webui/src/web-components/demo/index.html
index 2dd8f18..cc5697e 100644
--- a/webui/src/web-components/demo/index.html
+++ b/webui/src/web-components/demo/index.html
@@ -7,7 +7,7 @@
     <ul>
       <li><a href="sketch-app-shell.demo.html">sketch-app-shell</a></li>
       <li><a href="sketch-chat-input.demo.html">sketch-chat-input</a></li>
-      <li><a href="sketch-diff-view.demo.html">sketch-diff-view</a></li>
+
       <li>
         <a href="sketch-container-status.demo.html">sketch-container-status</a>
       </li>