Implement tag management with SQLite; update save and retrieve endpoints

public/js/tags.js

@@ -1,10 +1,13 @@
-export function saveTags(tags) {
+export async function saveTags(newTags) {
+    const existingTags = await loadTags();
+    const allTags = Array.from(new Set([...existingTags, ...newTags]));
+
     return fetch('/save-tags', {
         method: 'POST',
         headers: {
             'Content-Type': 'application/json',
         },
-        body: JSON.stringify({ tags })
+        body: JSON.stringify({ tags: allTags })
     });
 }