web: base custom instance functionality

also:
- renamed processing tab in settings to "instances"
- improved override description
- prefer custom over override (and grey out the option)
- dedicated lib for all api safety warnings
- left aligned small popup with smaller icon
- ability to grey out settings category & toggle

web/src/lib/api/api-url.ts

@@ -4,8 +4,16 @@ import env, { apiURL } from "$lib/env";
 import settings from "$lib/state/settings";
 
 export const currentApiURL = () => {
-    if (env.DEFAULT_API && get(settings).processing.allowDefaultOverride) {
+    const processingSettings = get(settings).processing;
+    const customInstanceURL = processingSettings.customInstanceURL;
+
+    if (processingSettings.enableCustomInstances && customInstanceURL.length > 0) {
+        return new URL(customInstanceURL).origin;
+    }
+
+    if (env.DEFAULT_API && processingSettings.allowDefaultOverride) {
         return new URL(env.DEFAULT_API).origin;
     }
+
     return new URL(apiURL).origin;
 }