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/routes/settings/instances/+page.svelte

@@ -0,0 +1,43 @@
+<script lang="ts">
+    import env from "$lib/env";
+    import settings from "$lib/state/settings";
+
+    import { t } from "$lib/i18n/translations";
+
+    import SettingsToggle from "$components/buttons/SettingsToggle.svelte";
+    import SettingsCategory from "$components/settings/SettingsCategory.svelte";
+    import CustomInstanceInput from "$components/settings/CustomInstanceInput.svelte";
+
+    $: overrideDisabled = $settings.processing.enableCustomInstances;
+</script>
+
+{#if env.DEFAULT_API}
+    <SettingsCategory
+        sectionId="override"
+        title={$t("settings.processing.override")}
+        disabled={overrideDisabled}
+    >
+        <SettingsToggle
+            settingContext="processing"
+            settingId="allowDefaultOverride"
+            title={$t("settings.processing.override.title")}
+            description={$t("settings.processing.override.description")}
+            disabled={overrideDisabled}
+        />
+    </SettingsCategory>
+{/if}
+
+<SettingsCategory
+    sectionId="community"
+    title={$t("settings.processing.community")}
+>
+    <SettingsToggle
+        settingContext="processing"
+        settingId="enableCustomInstances"
+        title={$t("settings.processing.enable_custom.title")}
+        description={$t("settings.processing.enable_custom.description")}
+    />
+    {#if $settings.processing.enableCustomInstances}
+        <CustomInstanceInput />
+    {/if}
+</SettingsCategory>