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/settings/defaults.ts

@@ -33,7 +33,10 @@ const defaultSettings: CobaltSettings = {
     },
     processing: {
         allowDefaultOverride: false,
+        customInstanceURL: "",
+        enableCustomInstances: false,
         seenOverrideWarning: false,
+        seenCustomWarning: false,
     }
 }