feat: Web-Search Sources anzeigen (Streaming + History + Archive + Shared)

Backend:
- web_search_tool_result Sources extrahieren (url, title)
- Als 'web_search_result' Message-Typ in DB persistieren
- Sources im web_search_end SSE-Event mitsenden
- Alle 3 Chat-Pfade: StreamChat, RunAgentChat, ChatNonStreaming

Frontend:
- renderToolBoxContent: Sources-Sektion mit klickbaren Domain-Links
- finishToolBox/addToolBoxStatic: sources-Parameter durchgereicht
- History: web_search_result Message-Typ in allen 3 Render-Pfaden
- web_search immer sichtbar (auch bei 'modifications' Filter)

CSS:
- .web-search-sources: Flex-Wrap Container
- .web-search-source: Domain-Pill mit Hover-Effekt, Title-Ellipsis

wwwroot/css/chat.css

@@ -170,6 +170,55 @@
   padding: var(--badge-padding);
 }
 
+/* Web Search Sources */
+
+.web-search-sources {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.4em;
+  padding: var(--box-padding);
+  padding-top: 0;
+}
+
+.web-search-source {
+  display: inline-flex;
+  align-items: center;
+  gap: 0.3em;
+  padding: 0.2em 0.6em;
+  background: var(--f2-bg);
+  border: var(--border-width) solid var(--f2-border);
+  border-radius: var(--border-radius);
+  color: var(--text) !important;
+  text-decoration: none !important;
+  font-size: 0.85em;
+  transition: background var(--transition-speed), border-color var(--transition-speed);
+  max-width: 20em;
+  overflow: hidden;
+}
+
+.web-search-source:hover {
+  background: var(--primary);
+  border-color: var(--primary);
+  color: var(--primary-text) !important;
+}
+
+.web-search-source .fa {
+  font-size: 0.75em;
+  opacity: 0.6;
+}
+
+.source-domain {
+  font-weight: 600;
+  white-space: nowrap;
+}
+
+.source-title {
+  opacity: 0.7;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
 /* Diff View */
 
 .tool-diff {