Fix notification panel readability — dark theme styling

Replace hard-coded white/light colors with dark theme palette matching the rest
of the app. Key changes: dark navy background (#1e293b/#0f172a), high-contrast
text (#f1f5f9 titles, #94a3b8 body), teal accent (#14b8a6) for active filter and
unread left-border, red (#f87171) clear button, dark scrollbar, subtle hover
states on items and action buttons.

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

client/build/asset-manifest.json

@@ -1,15 +1,15 @@
 {
   "files": {
-    "main.css": "/static/css/main.40bbff16.css",
+    "main.css": "/static/css/main.5afb44a9.css",
     "main.js": "/static/js/main.a387bcf0.js",
     "static/js/453.d456ce70.chunk.js": "/static/js/453.d456ce70.chunk.js",
     "index.html": "/index.html",
-    "main.40bbff16.css.map": "/static/css/main.40bbff16.css.map",
+    "main.5afb44a9.css.map": "/static/css/main.5afb44a9.css.map",
     "main.a387bcf0.js.map": "/static/js/main.a387bcf0.js.map",
     "453.d456ce70.chunk.js.map": "/static/js/453.d456ce70.chunk.js.map"
   },
   "entrypoints": [
-    "static/css/main.40bbff16.css",
+    "static/css/main.5afb44a9.css",
     "static/js/main.a387bcf0.js"
   ]
 }