feat: add pin/unpin functionality to chat history

Adds pin/unpin functionality to the chat history sidebar, allowing users to keep important conversations readily accessible. This improves user experience and helps organize past interactions.

This feature includes:
- Pin/unpin buttons in the chat history sidebar.
- Updated database schema to include `is_pinned` field for chat history items.
- Localized translations for pin/unpin actions.
- Updated UI to display pinned items at the top of the list.

src/assets/locale/de/common.json

@@ -41,6 +41,7 @@
     "webSearch": "Web durchsuchen",
     "regenerate": "Neu generieren",
     "edit": "Bearbeiten",
+    "delete": "Löschen",
     "saveAndSubmit": "Speichern & Absenden",
     "editMessage": {
         "placeholder": "Nachricht eingeben..."
@@ -102,5 +103,14 @@
         "custom": "Benutzerdefiniert"
     },
     "citations": "Zitate",
-    "downloadCode": "Code herunterladen"
+    "downloadCode": "Code herunterladen",
+    "date": {
+        "pinned": "Angepinnt",
+        "today": "Heute",
+        "yesterday": "Gestern",
+        "last7Days": "Letzte 7 Tage",
+        "older": "Älter"
+    },
+    "pin": "Anheften",
+    "unpin": "Losheften"
 }