feat: Einzelne Nachrichten als Link teilen

Backend:
- SharedMessageRecord Model + shared_messages DB-Tabelle
- SharedEndpoint: POST /api/messages/{id}/share, GET /api/shared/{token},
  GET /api/shared (Liste), DELETE /api/shared/{token}
- Auth-Ausnahme für GET /api/shared/{token} (öffentlich ohne Login)
- DB-Methoden: CreateSharedMessage, GetSharedMessage, GetChainMessagesUpTo

Frontend:
- Share-Button (fa-share-alt) an jeder Nachricht
- Kontextmenü: Einzelnachricht oder mit Thread-Kontext teilen
- Link wird automatisch in Zwischenablage kopiert
- Öffentliche Shared-Page (#shared/{token}) ohne Login
- Markdown-Rendering in der geteilten Ansicht
- CSS für Share-Button, Share-Menü und Shared-Page

wwwroot/css/chat.css

@@ -414,7 +414,8 @@
 
 .msg-reply-btn,
 .msg-delete-btn,
-.msg-mark-btn {
+.msg-mark-btn,
+.msg-share-btn {
   position: absolute;
   top: var(--badge-spacing);
   opacity: 0;
@@ -428,20 +429,25 @@
 }
 
 .msg-reply-btn {
-  right: calc(var(--badge-spacing) + 4em + var(--badge-spacing));
+  right: calc(var(--badge-spacing) + 6em + var(--badge-spacing) * 1.5);
 }
 
 .msg-delete-btn {
-  right: calc(var(--badge-spacing) + 2em + var(--badge-spacing) / 2);
+  right: calc(var(--badge-spacing) + 4em + var(--badge-spacing));
 }
 
 .msg-mark-btn {
+  right: calc(var(--badge-spacing) + 2em + var(--badge-spacing) / 2);
+}
+
+.msg-share-btn {
   right: var(--badge-spacing);
 }
 
 .msg:hover .msg-reply-btn,
 .msg:hover .msg-delete-btn,
-.msg:hover .msg-mark-btn {
+.msg:hover .msg-mark-btn,
+.msg:hover .msg-share-btn {
   opacity: 1;
 }
 
@@ -858,3 +864,112 @@
   object-fit: contain;
   border-radius: var(--border-radius);
 }
+
+/* Share Menu */
+
+.share-menu {
+  background: var(--f2-bg);
+  border: var(--border-width) solid var(--f2-border);
+  border-radius: var(--border-radius);
+  overflow: hidden;
+  display: flex;
+  flex-direction: column;
+  box-shadow: 0 4px 16px rgb(0 0 0 / 0.25);
+  min-width: 12em;
+}
+
+.share-menu-item {
+  display: flex;
+  align-items: center;
+  gap: var(--badge-spacing);
+  padding: var(--menu-item-padding);
+  background: none;
+  border: none;
+  text-align: left;
+  white-space: nowrap;
+  cursor: pointer;
+  color: var(--text);
+  font-size: var(--font-size);
+}
+
+.share-menu-item:hover {
+  background: var(--f3-bg);
+  border: none;
+}
+
+/* Shared Page (öffentliche Ansicht) */
+
+.shared-page {
+  display: flex;
+  flex-direction: column;
+  min-height: 100dvh;
+  max-width: var(--page-width);
+  margin: 0 auto;
+}
+
+.shared-header {
+  display: flex;
+  align-items: center;
+  gap: var(--box-spacing);
+  padding: var(--page-padding);
+  border-bottom: var(--border-width) solid var(--f2-border);
+}
+
+.shared-logo {
+  font-weight: bold;
+  font-size: 1.2em;
+  letter-spacing: .05em;
+}
+
+.shared-label {
+  opacity: var(--semi-transparent);
+  font-size: var(--font-size);
+}
+
+.shared-content {
+  flex: 1;
+  padding: var(--page-padding);
+  display: flex;
+  flex-direction: column;
+  gap: var(--badge-spacing);
+  overflow-y: auto;
+}
+
+.shared-content .msg {
+  max-width: 100%;
+}
+
+.shared-loading,
+.shared-error {
+  text-align: center;
+  padding: 3em 1em;
+  opacity: var(--semi-transparent);
+}
+
+.shared-error {
+  color: var(--dangerous);
+  opacity: 1;
+}
+
+.shared-meta {
+  text-align: center;
+  padding: var(--box-padding);
+  opacity: var(--semi-transparent);
+  font-size: var(--font-size);
+}
+
+.shared-footer {
+  padding: var(--page-padding);
+  text-align: center;
+  border-top: var(--border-width) solid var(--f2-border);
+}
+
+.shared-cta {
+  color: var(--primary);
+  text-decoration: none;
+  font-weight: bold;
+}
+
+.shared-cta:hover {
+  text-decoration: underline;
+}