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/mediawiki_oauth.py

@@ -1,5 +1,6 @@
 """Wikipedia OAuth."""
 
+import sys
 import typing
 import urllib
 from typing import cast
@@ -73,9 +74,8 @@ def api_request(params: typing.Mapping[str, str | int]) -> dict[str, typing.Any]
     try:
         return cast(dict[str, typing.Any], r.json())
     except Exception:
-        print("text")
-        print(r.text)
-        print("---")
+        print(f"API request failed: HTTP {r.status_code}", file=sys.stderr)
+        print(f"Response body: {r.text!r}", file=sys.stderr)
         raise
 
 
@@ -105,7 +105,13 @@ def get_username() -> None | str:
         return None  # not authorized
 
     if "username" not in session:
-        reply = userinfo_call()
+        try:
+            reply = userinfo_call()
+        except Exception as e:
+            print(f"get_username failed, clearing session: {e}", file=sys.stderr)
+            session.pop("owner_key", None)
+            session.pop("owner_secret", None)
+            return None
         if "query" not in reply:
             return None
         session["username"] = reply["query"]["userinfo"]["name"]