Redesign UI and fix several bugs

- Add navbar with login/logout, search form, and Find Link branding
- Clean up index page: search-only, examples behind ?debug=1
- Improve article page: remove debug clutter, named Wikipedia links, collapsible candidates
- Add SVG favicon (🔗 emoji)
- Fix diff CSS: compact layout, auto table layout to eliminate wide marker column gap
- Catch TokenRequestDenied in OAuth start and show error page
- Store username in session at login; clear bad session on API failure
- Raise NoMatch when diff is empty (edit already applied)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

add_links/match.py

@@ -559,4 +559,6 @@ def get_diff(q: str, title: str, linkto: str | None) -> dict[str, typing.Any]:
     )
 
     found["diff"] = call_get_diff(title, found["section_num"], section_text)
+    if not found["diff"]:
+        raise NoMatch
     return found