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>

static/css/diff.css

@@ -5,19 +5,16 @@ span.searchmatch { font-weight: bold; }
 
 table.diff,td.diff-otitle,td.diff-ntitle{background-color:white}
 td.diff-otitle,td.diff-ntitle{text-align:center}
-td.diff-marker{text-align:right;font-weight:bold;font-size:1.25em}
+td.diff-marker{width:1.5em;text-align:center;font-weight:bold;font-size:1.25em;padding:0 0.3em}
 td.diff-lineno{font-weight:bold}
 td.diff-addedline,td.diff-deletedline,td.diff-context{font-size:88%;vertical-align:top;white-space:-moz-pre-wrap;white-space:pre-wrap}
-td.diff-addedline,td.diff-deletedline{border-style:solid;border-width:1px 1px 1px 4px;border-radius:0.33em}
-td.diff-addedline{border-color:#a3d3ff}
-td.diff-deletedline{border-color:#ffe49c}
-td.diff-context{background:#f3f3f3;color:#333333;border-style:solid;border-width:1px 1px 1px 4px;border-color:#e6e6e6;border-radius:0.33em}
+td.diff-addedline,td.diff-deletedline{border-left:3px solid}
+td.diff-addedline{border-color:#a3d3ff;background:#f0f8ff}
+td.diff-deletedline{border-color:#ffe49c;background:#fffaf0}
+td.diff-context{color:#555}
 .diffchange{font-weight:bold;text-decoration:none}
-table.diff{border:none;width:98%;border-spacing:4px; table-layout:fixed}
-td.diff-addedline .diffchange,td.diff-deletedline .diffchange{border-radius:0.33em;padding:0.25em 0}
+table.diff{border:none;width:100%;border-spacing:0;border-collapse:collapse;table-layout:auto}
 td.diff-addedline .diffchange{background:#d8ecff}
 td.diff-deletedline .diffchange{background:#feeec8}
-table.diff td{padding:0.33em 0.66em}
-table.diff col.diff-marker{width:2%}
-table.diff col.diff-content{width:48%}
-table.diff td div{ word-wrap:break-word; overflow:auto}
+table.diff td{padding:0.2em 0.5em}
+table.diff td div{word-wrap:break-word;overflow:auto}

static/favicon.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+  <text y=".9em" font-size="90">🔗</text>
+</svg>